NASA/TP-2003-212140

InGaAsSb Detectors' Characterization for

CO 2 Lidar/DIAL Applications

Tamer F. Refaat

Science and Technology Corporation, Hampton, Virginia

M. Nurul Abedin, Grady J. Koch, and Upendra N. Singh

Langley Research Center, Hampton, Virginia

February 2003
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NASA/TP-2003-212140

InGaAsSb Detectors' Characterization for

CO 2 Lidar/DIAL Applications

Tamer F. Refaat

Science and Technology Corporation, Hampton, Virginia

M. Nurul Abedin, Grady J. Koch, and Upendra N. Singh

Langley Research Center, Hampton, Virginia

National Aeronautics and

Space Administration

Langley Research Center

Hampton, Virginia 23681-2199

February 2003
Acknowledgments

The authors would like to thank the Earth Sciences Technology Office (ESTO) at NASA Goddard Space Flight

Center for financial support of 2-gin detector development at NASA Langley Research Center. The authors would like

to extend their acknowledgments to Dr. Syed lsmail for his suggestions and discussions on the science of 2-gin Lidar/

DIAL remote sensing for monitoring the atmospheric constituents. Thanks also go to Oleg V. Sulima, Michael Mauk,

and Louis DiNetta of AstroPower, Inc. Their help and discussion about lnGaAsSb detectors are very much

appreciated.

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not constitute an

official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics and

Space Administration.

Available from:

NASA Center for AeroSpace Information (CAS1)

7121 Standard Drive

Hanover, MD 21076-1320

(301) 621-0390

National Technical Information Service (NTIS)

5285 Port Royal Road

Springfield, VA 22161-2171

(703) 605-6000
Abstract

Recent interest in monitoring atmospheric carbon dioxide Jocuses

attention on inJbared remote sensing using the 2-ym lidar/differential

absorption lidar (DIAL) technique. Quantum detectors are critical components in this technique, and many research efforts concentrate on

developing such devices Jor the 2-_tm wavelength. In this report, the

characterization results of InGaAsSb quantum detectors J or the 2-_tm

wavelength range are presented, including the experimental setup and

procedure. The detectors are prototype devices manufactured by using

separate absorption and multiplication (SAM) structures. The characterization experiments include voltage-current (V-I) measurements,

spectral response and its variation with bias voltage and temperature,

noise measurements, noise-equivalent-power (NEP) and detectivity calculations, and signal-to-noise ratio (SNR) estimation. A slight increase

in the output signal was observed by increasing the bias voltage, which

was associated with an increase in the noise level. Cooling down the

detectors reduces their noise and shifts the cutoff wavelength to shorter

values. Further improvement in the design and manufacturing process,

by increasing the device gain and lowering its noise level, is necessary to

meet the required C02 lidar/DIAL specifications.

1. Introduction

Light detection and ranging (lidar) is a remote sensing technique suitable for monitoring and mapping

atmospheric species. Among these species, carbon dioxide (CO2) recently has gained interest as one of

the dominant greenhouse gases. Kaiser and Schmidt discussed the problem of the CO2 natural cycle and

the uncertainties in understanding the atmospheric CO2 budget (ref. 1). The ability to monitor atmospheric CO2 in the shortwave infrared spectral region has been reported in several publications. Ambrico

et al. discussed the sensitivity of both 1.6- and 2-_tm differential absorption lidar (DIAL) measurements

and emphasized the advantage of the 2-_tm wavelength (ref. 2). Ismail et al. simulated CO2 measurements

by using the same technique (refs. 3 and 4), and Taczak and Killinger showed attempts for detecting CO2

using 2 _tm (ref. 5). Currently, at NASA Langley Research Center, research is focused on the development of differential absorption lidar (DIAL) technology for CO2 measurements at the 2-_tm wavelength

(refs. 3 and 4).

Among the different components of the lidar/DIAL system, the receiver optical detector is of special

interest. The detector converts the optical power of the lidar return signal into an electrical signal, thereby

directly affecting the instrument measurement sensitivity (ref. 6). Being the dominant noise source in the

lidar instrument, the detector affects the system minimum detectable signal and correspondingly the

minimum detectable concentration at a certain range (ref. 7). These issues drive the need for a detector of

narrow spectral bandwidth, high quantum efficiency, and low noise. Small area avalanche photodiodes

(APDs) are an attractive solution for lidar receivers due to their internal gain mechanism and relatively

low noise.

Although 2-_tm APDs are commercially unavailable, simulation showed that for successful lidar/DIAL

measurements, a detector noise equivalent power (NEP) level in the range of 10-14 W/Hz 1/2 is necessary

with 55 percent quantum efficiency and at least a 300-_tm diameter of the active area (ref. 4). Responsivity as high as 50 A/W, narrow spectral bandwidth, and room temperature operation are further advantages
of suchadevice.In thisreportwewill discussthecharacterizationof InGaAsSbbaseddetectorsfor2-gm

applications. ThesedevicesweremanufacturedbyAstroPower,Inc.(Newark,DE)usinga prototype

APDstructurewithseparateabsorptionandmultiplicationregions(SAM).Theexperimental workwas

completedtoinvestigatehesedevicesagainsttheCO2lidar/DIALmeasurement requirements.

1.1. Lidar/DIAL Techniques

The lidar technique uses a pulsed laser source tuned to an absorption feature of an atmospheric species. A collocated receiver telescope collects part of the atmospheric backscattered radiation, which

results in density measurements of the constituent as a function of range. The scattering lidar equation

used to correlate the transmitted laser power P(r) and the constituent number density N(r) as a function of

the range r has the form

P(r): poCZAqr_(r)exp{-2 Io [oN(r)+tc(r)]dr} 2r 2

(1)

where Po is the initial transmitted power, c is the speed of light, "c is the laser pulse duration, A is the

telescope receiver area, fir is the receiver efficiency, 13(r) is the atmospheric volume backscatter

coefficient, o is the absorption cross section of the molecular species, and _:(r) is the atmospheric

extinction coefficient (refs. 7 9).

Because the absorption profile is a unique characteristic for each molecular species, the DIAL technique can distinguish and monitor different atmospheric species such as CO2. In the DIAL technique, two

laser pulses are transmitted successively through the atmosphere. The first is tuned to a strong absorption

feature of the species (on-line) and the other is tuned to a weak or no absorption spectral region (off-line).

Assuming the two wavelengths are spectrally close (=100 pm), the atmospheric attenuation and scattering

will be equal for both wavelength pulses. Applying equation (1) to both the on- and off-line conditions

and solving for the number density yields

1 ln[Poff(r2)Pon(rl)-

N(r)= 2(r2- rl)((Jon-(Joff ) [Poff(rl)Pon(r2)

(2)

where the subscripts off and on are for the off-line and on-line wavelengths, respectively, (r2 - rl) is the

measurement range cell, and (Oon- Ooff) is the differential absorption cross section (ref. 9).

The optical detector at the lidar receiver telescope output is used in conjunction with a transimpedance

amplifier (TIA) to convert the collected radiation power into a voltage V according to

v(t):R: P(r) (3)

after correcting for the signal background, where R/is the TIA feedback resistance and 9_(X, VB, T) is the

detector responsivity, which is a function of the operating wavelength X, the detector operating bias

voltage VB, and temperature T. The time dependence of equation (3) is converted into spatial (range)

dependence by using the speed of light in the relation (ref. 10; to be published).

c.t

r = -- (4)

2
The factor 2 in the above equation accounts for the round-trip time for the transmitted and the collected

backscattered radiation, respectively. Substituting equation (3) into equation (2), the number density can

be evaluated directly from the detected signals and will be given by

= - )("on- ) "[ )Yon (5)

The DIAL technique can be applied to many atmospheric species, provided that suitable laser sources and

detectors are available. Besides CO2 lidar atmospheric measurements, the 2-gm wavelength is also

suitable for water vapor, wind, and cloud measurements.

1.2. InGaAsSb Detectors Overview

InGaAsSb is a promising semiconductor material for developing 2-gm APD detectors. Several articles

have discussed the properties of this kind of device using SAM structure (refs. 11-20). The avalanche

multiplication process in InGaAsSb material was studied by Andreev et al. (ref. 11). Multiplication

factors of 10 20 and 50 100 were achieved at 296 K and 78 K, respectively. Electron and hole impact

ionization coefficients were evaluated, and their empirical formulas were given as a function of the electric field (ref. 12). An ionization ratio (defined by the ratio of electron to hole ionization coefficients) up

to 60 was reported (ref. 13). Unlike silicon APDs, it was found that holes dominate the impact ionization

process, which leads to a relatively higher device noise (ref. 14; to be published). The relation between

the impact ionization coefficients and the excess noise factor was also discussed (ref. 12). The same group

studied a GaInAsSb/GaA1AsSb SAM APD structure and examined the avalanche breakdown by studying

the reverse bias part of the voltage-cuxrent characteristics (V-I) at different temperatures. The variation of

the spectral response profile with the bias voltage was also given. It was observed that the spectral

response peak shifts toward longer wavelengths with higher voltage (ref. 15). Generally, the InGaAsSb

material structure was found to have high dark current with mA range damage threshold.

Benoit et al. emphasized the constraints of the value and profile of the electric field for fast and sensitive devices and discussed dark current reduction (ref. 16). Voronina et al. studied the mobility of the

charge carriers and its dependence on the doping type and concentration and on the device temperature in

InGaAsSb material (ref. 17). Development of InGaAsSb p-n and SAM APD structures were discussed by

Shellenbarger et al. (refs. 18 20). Different characteristics were reported, based on frontal and backward

illumination of the devices. A backward illuminated p-n junction had a narrow spectral band centered

near 2 gm, which has the advantage of reducing the background signal and increasing the detector

dynamic range. Several other results were reported for p-n and PIN junctions of the same material

(refs. 21 24). These structures have lower noise and higher speed compared to APDs (ref. 25). Table 1

summarizes the detectivity (D*) measurements of some of these detectors as well as their quantum efficiency. By using the reported area, the NEP can be estimated as given in the table. Besides being under

development, none of the reported devices meet the 2-gm CO2 lidar/DIAL requirements (refs. 3 and 4).

1.3. Detectors' Structure and Operation

A schematic of the SAM APD structure is shown in figure 1. The SAM structure mainly consists of an

absorbing narrow bandgap InGaAsSb layer grown on a GaSb substrate, followed by a wide bandgap n

and p+A1GaAsSb multiplication layers. Separation of the absorption and multiplication layers helps produce high gain with relatively lower dark current (refs. 13, 15, 16, 18, and 20). The absorption layer width

is relatively larger than the multiplication layer, which increases the photon absorption probability

and enables the photo-generated charge carriers to reach their optimum velocity corresponding to the
maximumkineticenergy.Ontheotherhand,themultiplicationlayerhasarelativelyhigherelectricfield,

whichincreasestheimpactionizationprobabilityforreleasingadditionalchargecarriersthatleadtothe

deviceinternalgain.

Thedesignconstraintsof the SAMstructureweresummarizedby Benoitet al. aslistedin the

following:(1) Themaximumvalueof themultiplicationregionelectricfield mustbegreaterthan

2.7x 105V/cminorderto generateanefficientmultiplication;(2)forthecarrierstoreachtheiroptimum

velocityandcorrespondingmaximumenergy,theabsorptionlayermustbefullydepleted,anditsdepth

shouldbeatleasttwicetheabsorptionlength;(3)theelectricfieldattheabsorption-multiplication interfaceshouldbehigherthan5x 104V/cmto avoidchargecarrierpileup;and(4) themultiplieddark

currentmustbeminimized.An additionalwindowlayercanbeaddedto theradiationentrysidefor

enhancement of thesurfacereflectioncoefficient.Also,abackreflectivelayerallowstheunabsorbed

photonstoreenterthedevice,increasingtheabsorptionprobability.Generally,InGaAsSbSAMAPDsare

expectedtohavehighergainandhighernoisecomparedtoInGaAsSbp-nandPINdiodes(ref.16).

2. Experimental Setup

An experimental setup was assembled to characterize the prototype InGaAsSb detectors considered in

this research study. The setup was designed to control the operating conditions of the test detector in

terms of bias voltage and temperature and to apply a well-defined optical signal. Also, this setup acquires

the detector V-I characteristics, spectral response calibration, and dark noise measurement. Appendix A

lists the instruments used to assemble this setup.

Figure 2 shows a schematic of the setup instrumentation and arrangements (refs. 10, 26, and 27). The

optical section was designed to apply stable and uniform monochromatic radiation onto the detector in the

1- to 3.2-gm spectral range. The electrical section primarily measures the detector output while controlling its bias voltage and temperature. To minimize calibration errors, the setup has to accommodate for

switching between the test and reference detectors with a minimal change in the optical signal and with

minimal misalignment. An optical microscope was used to align both the reference and test detectors at

the same position with respect to the optical axis and the radiation source (refs. 10, 26, and 27). The

microscope had a 15X and 10X eyepiece and objective lenses, respectively, with a 9.5 ° field of view and

a 15.6-mm objective distance.

The investigated detectors were calibrated by using a 3 x 3 mm2 PbS reference detector and applying

the substitution method, which is discussed in section 4.1. The reference detector was calibrated in the

1- to 3.2-gm spectral range, with 100 nm steps, as shown in figure 3. The estimated uncertainty in the

reference detector transferred calibration is 1.0 percent for wavelengths up to 2.5 btm and 1.5 percent for

longer wavelengths. Another single point calibration was obtained at 1300 nm, relative to a National

Institute of Standards and Technology (NIST) calibrated Ge detector (ref. 28).

2.1. Optical Setup

A quartz halogen lamp, operated by a stabilized current supply, was used as the radiation source. An

optical chopper was used to modulate the radiation at 167 Hz (prime number frequency was selected to

minimize pickup noise). A monochromator separated the radiation into its spectral components. The

monochromator slit-widths were set to 1.25 mm, and two gratings were switched according to the scan

wavelength range. High-pass optical filters were installed at the monochromator outlet to block the higher

order dispersion of the shorter wavelengths (refs. 26 and 27). The grating and filters cut on wavelength as

a function of the scanning wavelength and the corresponding spectral resolution are given in table 2.
Beam-steeringopticsincreasedtheopticalpathto collimatetheradiationincidentonthedetector.Additionaloptics,suchasdiffusersandneutraldensityfilters,wereinstalledto fuxtherconditiontheradiation.

Thescanstepsizewasselectedtobe40nm,equivalent tothesetupmaximumresolution(ref.29).

Themonochromator gratingwastestedby usinganinfraredlasersource(appendixA).Thenarrow

linewidth(lessthan1pm)andpreciselinelocation(2.051gm)of thelaserallowcheckingof themonochromatortansfercharacteristicsandspectralcalibration,asshownin figure4.In thisexperiment, the

slit-widthsweresetto a0.5-mmequivalenttoaresolutionof4 nmupto2050nmand16nmforawavelengthlargerthan2050nm.Thescanwasobtainedbetween2000and2100nmwith5-nmsteps.The

asymmetrybetweentheleftandrightportionfromthepeakis dueto thechangeinthescanresolutionfor

thedifferentgratings.

2.2. Electrical Setup

The TIA circuit used for the detector's current-to-voltage conversion is shown in figure 5 (refs. 14

and 30). A low-noise operational amplifier was used to build this circuit (ref. 31). Resistor R is used to

limit the detector current. Also, R associated with the capacitor C acts as a low-pass filter to block highfrequency noise in the detector bias voltage. For zero bias voltage operation, both the resistor and capacitor were removed from the circuit. The feedback resistor Rfdefines the current-to-voltage conversion gain

according to

V=-Rf.I s (6)

where I S is the detector output current. The TIA and the detector were mounted on a small printed circuit

board (PCB) that is installed inside a chamber (refs. 10, 26, 27, and 32). The total noise contribution

of the TIA circuit was estimated to be 1.4 x 10-12 A/Hzl/2 referred to the output (refs. 14, 30, and 31).

Table 3 lists the noise contribution for each circuit component as estimated at room temperature. A stable

high-voltage power supply biased the APD, and a +15-V power supply was used to bias the TIA. A

drawback of this circuit arises from the dc offset at the output due to the detector dark current that

saturates the operational amplifier. In this case, the TIA is replaced by a low-noise current preamplifier

(appendix A) that injects a countercurrent that balances the detector dark current.

The TIA output was connected to an amplifier support module (ref. 29) that further amplifies the signal. The ac coupling was used to eliminate the dc voltage component that resulted from the detector dark

current and amplifier offset. A lock-in amplifier was used to measure the detected signal peak-to-peak

value, and an oscilloscope was used to check the signal variation. A personal computer (PC) sent commands to the monochromator to adjust the grating position that sets the wavelength for the spectral scan.

The PC also acquired the lock-in amplifier reading at every scan step, adjusted the chopping frequency,

and supplied synchronizing signals to the other instruments.

2.3. Detector Chamber

Figure 6 shows the assembly diagram of the detector chamber (ref. 32). The chamber is designed to

control the detector operating environment and provide a mechanical mount for alignment. Two

thermoelectric coolers (TEC) cool the detector mounting plate. The chamber wall acts as a heat sink for

the TEC, and water circulation at the chamber back plate dissipates the heat. A refrigerated recirculator

provides water circulation at 4 °C. Nitrogen purging prevents water vapor condensation and ice formation

on the detector window at low temperatures and also prevents dust accumulation inside the chamber.

Detector temperature is monitored by a calibrated thermistor, which is located on the mounting plate as
closeaspossibletothedetector.A temperaturecontrollerisusedtocontroltheTECoperationandalsoto

provideatemperatureadout(ref.32).Thisconfigurationallowscoolingdownthedetectorto 20°C,

with0.1and0.01°Ctemperaturesettingandreadoutresolutions, respectively. Thechamberwaslocated

onathree-axistranslationstageforalignmentpurposes.

3. V-I Characteristics

The V-I characteristic is a relation between the detector current and its applied bias voltage. The characteristic shifts down by illuminating the device, satisfying the equation (refs. 25 and 33)

,: ] (7)

where I is the detector total current, Id is the detector dark current, Io is the saturation dark current, q is the

electron charge, VB is the detector bias voltage, K is Boltzmann's constant, and T is the operating

temperature. The detector current component that corresponds to the input radiation Is is given in terms of

the detector responsivity and incident optical power P in the relation

Is: (8)

The V-I characteristic is particularly important near detector breakdown because varying the temperature

can identify the breakdown mechanism. The breakdown voltage decreases by decreasing the temperature

for the avalanche breakdown mechanism (ref. 33); therefore, by obtaining the reverse branch of V-I

characteristics and observing the breakdown behavior, we can distinguish whether the detector is an APD

or a regular photodiode (refs. 20 and 33).

The V-I curves were obtained for the detectors under study by using a modular dc source/monitor.

Although the manufacturer did not specify the breakdown voltage, the measurements were nondestructive, as was examined by reproducing low-voltage results before and after increasing the bias voltage to

near the breakdown level. The voltage scan starts at 100 mV forward bias, with a step of 10 mV toward

the reverse bias. The reverse bias was selected carefully to limit the device current to 100 gA. Figure 7

shows the V-I characteristics of four different InGaAsSb samples at three different temperatures and three

samples at room temperature. A general observation for this material structure is the increase in the dark

current (abrupt in some cases) with reverse bias voltage, which reduces the dynamic range of the detector.

The current limit was exceeded for the detector D19-K204-11#6 (fig. 7(d)) up to 500 gA, corresponding

to a power consumption of 12.9 mW at 20 °C for power rating investigation. Cooling down this device to

20 °C reduces the dark current, leading to a lower power consumption of 6.5 roW. None of the examined

devices showed avalanche behavior, as is clear from the divergence of the V-I curves at higher voltage

and different temperatures. These results lead to the conclusion that none of the tested devices was an

APD.

4. Spectral Response Calibration

The spectral response is a relation between the detector responsivity in A/W or V/W and the wavelength of the incident optical signal at constant bias voltage and temperature. Generally, the detector

responsivity is directly proportional to the incident wavelength up to a point at which it begins to roll off.

Ideally, the roll-off is sharp and exists at a wavelength corresponding to the bandgap energy of the semiconductor material in the absorption layer (refs. 25 and 34). Referring to equations (7) and (8), detector
responsivitycanbedeterminedbymeasuringthedeviceoutputcurrentthatresultsfromaknownoptical

power.Thisoutputcurrenthasto bedetermined(aftersubtractingthedarkcurrent)at constantwavelength,biasvoltage,andtemperature. Usually,accouplingis usedto eliminatethedarkcurrentafter

modulatingtheopticalsignal.Modulationalsohelpsinmeasuringverylowsignalswithsensitiveinstruments,suchaslock-inamplifiers.Measuringtheinputmonochromaticsignalisacomplicatedtask,which

requireseitheracalibratedsourceoranothercalibratedreferencedetector.

4.1. Calibration by the Substitution Method

The calibration of the detectors under test was obtained by using the substitution method. In this

method, the spectral response of a calibrated reference detector is transferred to the test detector by

comparing the output of both devices with respect to a constant radiation source at a certain wavelength

(ref. 35). Referring to equation (8), assuming a radiation source with constant intensity Z (W/m2), the reference detector output Vr corresponding to this source will be equal to

Vr = 9_r" Ar "Z (9)

where 9_r is the reference detector responsivity (V/W). If a test detector with unknown responsivity

9_t (A/W) were to be placed in the same position as the reference detector with respect to the radiation

source, the output current It would be given by

It = _t At )_ (10)

assuming that both the bias voltage and the temperature of the test detector are constants. By dividing

equation (9) by equation (10) and referring to equation (6), the test detector responsivity will be given by

It" Rf A r 9_r

_t = Vr " At

(11)

where A r and A t are the reference and test detectors' areas, respectively. It should be noted that the

negative sign in equation (6) was neglected due to the ac coupling (i.e., the negative sign will be

equivalent to a phase shift of_).

The above equation can be modified to

I t .Rf Ar

9_t =CF 9_r (12)

Vr At

where CF is a correction factor. The correction factor accounts for any different measurement conditions

between the test and the reference detectors or their setups. In our case, the PbS reference detector has an

A r = 3 x 3 mm2 active area. The spectral response of this detector, given in figure 2, was obtained by

illuminating only a 1.5-mm diameter area of the device (equivalent to A'r) (ref. 28). Since the whole

sensitive area of the device is illuminated during the test detectors' calibration, an area correction factor

must be considered. Assuming the reference detector sensitive area is uniform, the correction of the

reference detector responsivity will be given by

CFA _ Ar _ 16 (13)

A'r
Finally, the detector quantum efficiency r I can be evaluated from the relation

hc 9t

n - (141

q G)_

where h is Planck's constant, c is the speed of light, q is the electron charge, and G is the detector gain, if

any.

4.2. Calibration Results

By using the experimental setup discussed above, the test detectors were calibrated, all with similar

operating conditions of 0-V bias at 20 °C temperature. The scan step was fixed at 40 nm with scan limits

of 1000 nm to 2400 nm. The data acquisition was set to 0.5- and 1-sec settling and integration times,

respectively, for high signal levels and equal settings of 2 sec for lower signal levels. An intensity monitor

is needed in such a setup to correct the data for the radiation source output variation. To avoid this problem, a warm-up time of 1 hr for the radiation source was considered to ensure stable operation. Also, the

reference detector measured the radiation source intensity before and after setting each of the test detectors. Intensity variation of less than 1 percent was verified before the calibration measurements.

Figure 8 shows the calibration results of the test detectors, assuming a constant diameter of the sensitive area of 200 gm for all the samples. A cutoff wavelength of about 2.2 gm was observed for all tested

devices, which corresponds to the bandgap energy of the composite material In0.15Ga0.85As0.17Sb0.83

(ref. 18). Some samples showed a narrower spectral period, corresponding to longer cut-on wavelength

(figs. 8(b) and 8(e)), which might be caused by a variation in the InGaAsSb layer thickness that affects

the photon absorption depth. The change in the spectral period width was reported for this material structure, and it was explained by the difference between frontal and backward illumination (ref. 18). This

narrow spectral period peaks around 2 gm, which is an advantage for the CO2 lidar/DIAL due to the

reduction of the background returns in the absence of a filter. Moreover, detector D13-K203-19#1 showed

two separate spectral periods that peaked at about 1 and 2 gm.

It was found that the magnitude of the detectors' responsivity is higher than possible in some cases.

This result was investigated by calculating the quantum efficiency by using equation (14) for all the samples, as shown in figure 9, assuming unity internal gain G. Quantum efficiencies higher than 100 percent

indicate two possibilities. Either the detectors exhibit some internal gain, or the constant diameter is not a

valid assumption for all samples. The second possibility has a higher probability because some of the

samples had problems with their contact beside the existence of an epoxy layer on top of the detectors'

sensitive area. It was reported that this epoxy layer had caused some problems in the detectors' calibration

due to its absorption characteristics around the 2-gm wavelength (ref. 19).

4.3. Spectral Response Variation With Bias Voltage

The detector responsivity is a function of the operating wavelength, bias voltage, and temperature, as

indicated in equation (3). At a certain wavelength and operating temperature, the spectral response variation with bias voltage can be investigated by using the substitution method. This method can be applied to

the same detector by knowing its calibration because the responsivity is directly proportional to the output
current.As anexample,knowingtheresponsivityatthecalibrationconditions9_c,withthecalibration

biasVc, the responsivity 9_x, at a different bias Vx, can be obtained from (refs. 10, 26, and 27)

--

(15)

where Ic and Ix are the detector output currents due to the bias voltages Vc and Vx, respectively.

By applying this method, the variation of the detectors' spectral response with the bias voltage was

obtained and is shown in figure 10. A room temperature of 20 °C was set for all the detectors. No abrupt

increase in the responsivity was observed at higher voltages, which confirms that the samples do not have

avalanche gain. Instead, a small increase was observed in some samples, which was associated with relatively higher noise levels. Enhancement of the narrow spectral period between 1.8 and 2.2 Bm was seen

for the samples D13-K203-19#1 and D19-K204-11#6. These interesting results not only confirm that the

detectors exhibit a certain internal nonavalanche gain, but that this gain is selectively applied to a certain

part of the detector's spectral range.

4.4. Spectral Response Variation With Temperature

Similarly, at a certain wavelength and operating bias voltage, the spectral response variation with temperatuxe can be investigated by using the substitution method. Knowing the responsivity at the calibration

conditions 9_c, with the calibration temperature Tc, the responsivity 9_x, at a different temperature Tx, can

be obtained from (refs. 10, 26, and 27)

(16)

where Ic and Ix are the detector output currents due to the temperatures Tc and Tx, respectively.

The variation of the detector's spectral response with the temperature is shown in figure 11. Cooling

down the detectors leads to a shift in the cutoff wavelength to a shorter value. This result was expected

because of the variation in the energy bandgap of the InGaAsSb with temperature (ref. 36). For the

detector samples D13-K203-19#1, D15-K204-13#8, and D20-K200-2, the spectral period (1.8 to 2.2 Bm)

shifts to a shorter value. Away from the spectral period, the three samples showed different behavior with

temperature. The responsivity of sample D13-K203-19#1, D15-K204-13#8, and D20-K200-2 had an

increase, no change, and a decrease with temperature, respectively.

5. Noise Characterization

Optical detector noise is divided into two types, synchronized and inherent. Synchronized or systematic noise is primarily caused by the electromagnetic and electrostatic interference between the detector

and its associated and surrounding circuitry. This type of noise is independent of the detector operation,

and it can be reduced successfully by proper grounding, shielding, and differential measurements. Inherent noise is associated with the detector operation and is caused by random processes and can therefore be

reduced by averaging. There are several types of inherent noise, the most dominant of which are shot

noise, dark current noise, background noise, thermal noise, and 1/f noise (refs. 34, 37, and 38). These

noise sources are discussed in the following section.
5.1. Inherent Detector Noise

For an optical detector, shot noise is generated because of the random arrival of the photons that correspond to the optical signal, which leads to random generation of the charge carriers and results in random

fluctuation in the detector current. If an internal gain exists, such as in the case of APDs, an additional

noise will be contributed due to the randomness in the gain process. This noise is known as the multiplication gain noise. The combined noise effect can be described in the mathematical form

ish°t) = 2qBG. 9_. P (17)

where B is the detector bandwidth.

The second term of equation (7) defines the detector dark current, which contributes a dc offset to the

detector output. Dark current mainly is due to the thermal generation of charge carriers in the depletion

region and their drift due to the existence of the electric field. Both processes are random in nature and

independent of the optical signal. Besides, these "dark" charge carriers will be further amplified by the

internal gain mechanism; therefore, the dark current noise will be dependent on the dark current and the

device internal gain and is given by

(idark)2: 2qBG. ]d (18)

Thermal noise (Johnson noise) is caused by the random motion of electrons in resistors. Therefore,

every resistor or wire will contribute to this noise, especially the TIA feedback resistor, leading to an increase in the detector's total noise. Johnson noise is given by

(/johnson) 2 4KTB - _- (19)

At low frequency, flicker or 1/f noise is observed in semiconductor materials. This noise is mainly

caused by the trap levels within the material bandgap; l/f noise is given empirically by

(iflicker) 2 a (20)

where F is the operating frequency, o_ is a constant close to unity, and a is a constant which may depend

on the current through the detector and the bandwidth (refs. 35 and 37).

All objects emit blackbody radiation. If this radiation lies in the detector spectral range, the detector

will respond to it because it cannot distinguish the signal from this radiation. To the detector, the background will be treated as a "signal" and equation (14) will apply, but with the background signal power

PB. The noise current in this case will be given by

(iback)2 = 2qBG. 9_. pB (21)

10
Thebackgroundpowercanbe estimatedfromthe Stephan-Boltzmannlaw (ref.35).Althoughthe

backgroundnoiseisnottrueinherentnoisebecauseit isdependent onthebackgroundradiation,it hasto

beconsideredsinceit isassociatedwiththedetectoroperation.

Thetotaldetectornoiseisthepowersumof theseparateindependent noisesourcesandwill begiven

by

In xf-B = _(ish°t) 2 + (idark) 2 +_1n/"J°hns°n'2) +_ln/"flicker _2(iback)2) + (22)

where In is the detector noise current in A/Hzl/2. The measured noise current from the detector circuit,

such as that described in figure 5, includes the detector and the TIA circuit noise, ITIA The detector

noise can be separated from the total noise /total by using the relation

I n = _(/t°tal)2 -- (/TIA) 2

(23)

The TIA noise can be obtained separately by operating the circuit and measuring its noise with the

detector replaced by a short circuit. The advantage of this method is that all the synchronized noise from

power supplies, instrumentation, and other sources will be included in this measurement.

5.2. Noise Measurement

Using equation (23), the noise spectral density was measured for the test detectors. The measurements

were obtained in dark condition by using different bias voltage and temperature settings. At a certain

setting, the noise voltage spectral density of the test detectors was measured in conjunction with the TIA

circuit by using a spectrum analyzer (appendix A) in the frequency band 1 Hz to 100 kHz, with 1-Hz

normalization and 50-kHz center frequency. The noise voltage was then converted to a noise current

spectral density using equation (6). The noise spectrum was averaged 1000 times with 50 _ termination,

and the mean value was calculated in the operating frequency band. The TIA noise current was measured

associated with the setup and found to be 50 x 10-12 A/Hzl/2, with the detector replaced by a short

circuit. The net detector noise current was then calculated by subtracting the TIA and setup noise from the

total noise measured with the detector.

For the InGaAsSb material structures under investigation, noise increases with increasing device

temperature and bias voltage. Figure 12(a) shows the variation of the noise current spectral density with

bias voltage obtained at 20 °C. Measurement discrepancy between the different samples was observed

and might be caused by poor ohmic contact in the packaging process. Figure 12(b) shows the noise

current spectral density variation with temperature obtained at 6- and 15-V bias for detector samples

D13-K203-19#1 and D19-K204-11#6, respectively, and 1-V bias for the other samples. Cooling down the

detector reduces the noise slightly. Although cryogenic cooling might provide better noise performance, it

was not used because of the undefined behavior of the epoxy at low temperatures. It should be noted that

the noise results do not include the signal shot noise, since dark condition has been used.

5.3. Noise-Equivalent-Power and Detectivity

Except for the shot noise, the noise sources discussed in section 5.1 are independent of the incident

optical power and can be considered constant for certain operating conditions. Shot noise is directly

11
proportionalto thesquarerootof theopticalpower.Theamountof theopticalpowerincidentona

detectorthatproducesanoutputsignalthatisequaltothenoisesignaldefinesthenoise-equivalent-power

(NEP).In otherwords,NEPis theopticalpowerthatproducesaunitysignal-to-noiseratio(SNR).For

betterdetectorcomparison, theNEPisnormalizedto abandwidthof 1Hzandis givenby (refs.34,37,

and38)

NEP=In (24)

9t

The detectivity of a detector (D*) is an efficient figure of merit when comparing detectors with different areas. The D* is defined by the reciprocal of the NEP normalized to the detector sensitive area A, and

is given by (refs. 34, 37, and 38)

D*- _ (25)

NEP

By characterizing the detector noise currents, the calibration results were used to estimate its NEP and

D*. Figure 13(a) shows the test detectors' NEP variation with wavelength obtained at the calibration conditions. Although the figure indicates the similar noise contents of some of these samples compared to

similar devices reported in the literature (table 1), none of the samples was close to the CO2 DIAL

requirement of 2 x 10-14 W/Hz 1/2 (ref. 4). The large NEP variation between the samples might be caused

by the nature of the material composition and/or the different device structures. This fact is consistent

with the results of figure 13(b), where D* is plotted against wavelength for the same detectors.

6. Signal-to-Noise Ratio

The detector power SNR is defined as the square of the signal current (or voltage) divided by the

square of the noise current (or voltage). Many factors affect the SNR of an optical detector, including its

operating bias voltage and temperature. As discussed earlier, decreasing the temperature of a detector had

almost no effect on its responsivity but slightly reduces the noise. Thus, we expect that cooling down the

detector will slightly enhance the SNR. On the other hand, increasing the bias voltage increases both the

detector responsivity and noise, which might enhance or deteriorate the SNR, depending on the rate by

which they individually increase.

Experimentally, SNR can be measured by applying a chopped optical signal to the detector. In this

case, the detector output will be similar to the signal shown in figure 14, in which Vd is the dark current

component corresponding to the "light off'' condition since it is blocked by the chopper blades, and Vs is

the signal current corresponding to the "light on" condition. The peak-to-peak optical signal, Vpp, is

defined as

Vpp = Vs - Vd (26)

Therefore, the power signal-to-noise ratio will be given here with respect to the rms or the standard

deviation of the signal, std (Vs):

SNR=[ Vpp ]2

[std(Vs)J

(27)

12
Figure15showsthevariationof the SNRwith thebiasvoltageandtemperature,obtainedat

2-gmwavelengthby usingconstantsourceradiationforthetwo detectorsamples,D20-K200-2and

D19-K204-11#8. Enhancement of theSNRwithdecreasingtemperaturewasobservedfor bothdevices.

Ontheotherhand,SNRdeterioratedwithincreasingbiasvoltage.

7. Concluding Remarks

The InGaAsSb material structure is a good candidate for 2-gm CO2 lidar detection. This report

presents the characterization results of prototype InGaAsSb detectors near the 2-gm wavelength region.

The characterization experiments include voltage-cuxrent (V-I) measurements, spectral response and its

variation with bias voltage and temperature, noise measurements, noise-equivalent-power (NEP) and

detectivity calculations, and signal-to-noise ratio (SNR) estimation. The results were not repeatable for all

the characterized detector samples. The temperature dependent V-I measurements indicated the detectors'

relatively high dark current, which decreases with temperature. No avalanche behavior was observed near

the detector breakdown. Spectral response measurements indicated that an optimization occurred in some

detectors around the 1.8- to 2.2-gm spectral region and peaked at 2 gm. All samples showed a cutoff

wavelength of 2.2 gm, with different cut-on wavelengths. Quantum efficiency calculations indicated

higher responsivity measurements in some spectral ranges, which might be caused by a lack of accuracy

in determining the detectors' sensitive area. Bias voltage slightly increases the responsivity only in a certain spectral range, thus confirming the absence of avalanche gain for these samples. Cooling the detectors leads to a shift in the cutoff wavelength to shorter values. Noise measurements showed slight

improvement in the noise current at lower temperatures. A significant increase in noise took place while

biasing the detectors.

Although the estimated NEP was comparable to similar, previously reported devices, the CO2 DIAL

system requires 2 x 10-14 W/Hz 1/2, which is about two orders of magnitude lower than the estimated

values. Thus, none of the detector samples is suitable for this application. Extensive research efforts are

required to improve the performance of the InGaAsSb detectors for DIAL system applications. These

efforts should focus mainly on minimizing the device noise and investigating the internal gain mechanism

that increases the signal level with bias voltage.

13
Appendix A

Instrumentation

The instruments used for detector characterization are as follows:

Manufacturer Model number Description

Boonton 7200 Capacitance meter

Hewlett Packard 4142B Modular dc source/monitor

Stanford Research Systems SR 785 2-Channel dynamic signal analyzer

Agilent Technologies 54845A Infiniium oscilloscope

Optronics Laboratories, Inc. 750-75MA Detector enclosure

Optronics Laboratories, Inc. 750-S Monochromator

Optronics Laboratories, Inc. 740-20 D/IR IR/Visible dual source attachment

Optronics Laboratories, Inc. 750-HSD-340 PbS detector head

Optronics Laboratories, Inc. DSM-2B Detector support module

Optronics Laboratories, Inc. 65A Programmable current source

Optronics Laboratories, Inc. 750-C Data acquisition and controller

Hewlett Packard E3632A DC power supply

Agilent Technologies E3631A Triple output power supply

ILX Lightwave LDC-3742B Laser diode controller

ILX Lightwave LMD-4412 Laser diode mount

Wavelength Electronics LFI-3751 Temperature controller

Neslab CFT-25 Refrigerated recirculator

Fluke 5111 Thermometer

Hewlett Packard E2373a Multimeter

Stanford Research Systems SR570 Low-noise current preamplifier

CLR Photonics, Inc. PZT METEOR single frequency laser

14
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16. Benoit, J.; Boulou, M.; Soulage, G.; Joullie, A.; and Mani, H.: Performance Evaluation of GaA1AsSb/GaInAsSb

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23. Li, A.; Zhong, J.; Zheng, Y.; Wang, J.; Ru, G.; Bi, W.; and Qi, M.: Molecular Beam Epitaxial Growth, Characterization and Performance of High-Detectivity GaInAsSb/GaSb PIN Detectors Operating at 2.0 to 2.6 gm. or. of

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Avalanche Photodiode Structures. Opt. Eng., vol. 39, no. 10, 2000, pp. 2642 2650.

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33. Sze, S.: Physics of Semiconductor Devices. John Wiley & Sons, Inc., 1981.

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16
35. Larason, Thomas C.; Bruce, Sally S.; and Parr, Albert C.: Spectroradiometric Detector Measurements:

Part I_ltraviolet Detectors and Part II_isible to Near-Infrared Detectors. NIST Spec. PubI., Feb. 1998,

pp. 250 41.

36. Yuan, T.; Baolin, Z.; Yixin, J.; Tianming, Z.; Shuwe, L.; Yongqiang, N.; Jinshan, Y.; Hong, J.; and Guang, Y.:

Calculation on Relation of Energy Bandgap to Composition and Temperature for GaxInl_xASl_ySby. Rare Metals, vol. 15, no. 3, 1996, pp. 172 178.

37. Vliet, K.: Noise Limitations in Solid State Photodetectors. AppI. Opt., vol. 6, no. 7, 1967, pp. 1145 1168.

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17
Table 1. Different Reported Detectors and Their Parameters: Active Area (A), Detectivity (D*),

Quantum Efficiency (QE), and Wavelength )_

Material structure

D* NEP

cm 2 cmHzl/2/W W/Hzl/2

p-GaInAsSb/nGaSb (p-n) 2.5 x 10 .4 7 x 109 2.3 x 10-12

p+GaInAsSb/p+GaInAsSb/n-GaSb 109

1.6 x 10 -3 1.3 x 10 -11

p+GaInAsSb/p-GaInAsSb/n+GaInAs 4 x 10 .4 3 x 109 6.7 x 10 -12

Sb/n+GaSb 7.85 x 10 .5 2.9 x 10 -12

pGaInAsSb/nGaInAsSb/nGaSb 3.61 x 10 .2 1.85 x 108 1.0 x 10-9

QE )_

Ref.

% gm

67 + 5 2.2 21

2.25 22

65 2.5 22

18

Table 2. Gratings and Filters Used as Function of Scan Spectral Range

With Corresponding Resolution

SpectrN range

Start, nm End, nm

Grating

grove/mm

Filter cut-on, nm

Resolution

FWHM a, nm

1000 1100 600 602 10

1110 1930 600 1119 10

1940 2040 600 1949 10

2050 3200 150 1949 40

aFull width at half maximum.

Table 3. Transimpedance Amplifier (TIA) Circuit Component

Values and Noise Contribution

Component Value Noise current

R l0 k_ 1.3 x 10 -12 A/Hz 1/2

C 1 BF

Rf 50 k_ 5.7 X 10 -13 A/Hz 1/2

Op amp OPA627 1.6 x 10 -13 A/Hz 1/2

18
Radiation

entry side

Metal contact

*--- Z p _-AIGaAsSb ..... -- - n-A1GaAsSb

n-InGaAsSb

n-GaSb

Metal contact

Figure 1. Schematic of InGaAsSb detector structure.

Chiller

Nitrogen

Power

supply (dc)

Enclosure box

1........................... Optical

[ Detector chopper

I chamber Ontional Beam Output J'N Input .....

ll_ o_ptics steering slit Cut-onfilter Monochromator /_ slit Ksa°_tca°n

i ..... " I .... ! _,_._

............ i.... ...... iI ,

i I \Testdetector[ Temperature .... I I F Current I

i I , controller Uscmoscope _ Chopper I I controller I

i _ I , r_ I I controller ] L_ I

'__ ._] ..................... _' __._ _o__________o____ _ i _ Computer

Amplifier I _ I

j _ ] [ Lock-in amplifier _ _ IL Jl

/-i.j / [ and control ] [ '"_"' [

Figure 2. Experimental setup for detector calibration.

× 10 5

6

4

;_ 3

2

_rL'>Y \

°\

\

1000 1500 2000 2500 3000 3500

Wavelength, nm

Figure 3. Spectral response calibration of PbS reference detector.

19
10-1

10-2

g 10-3

O,

10 -4

10-5

10---6

Oo 0

0

0

0

0

0

0

0

O Q ':D 0 Q O O 0

2000 2020 2040 2060 2080 2100 2120

Wavelength, nm

Figure 4. Reference detector output corresponding to 2.051-gm laser source.

Rf

Bias ............. 1_ I _-""'_ I

voltage R ,,tgetector[ b,_ [_,

_a

CT, ; _)pamp

Figure 5. Detector transimpedance amplifier (TIA) circuit.

Heat sink back plate _ h222:z:z_ Cooling water

/

Mounting screw

Thermistor

TIAcircuit_ _ _ _

Detector _ _-_ i__ _'_ _ _ t

Chamber wall --

Mounting plateTEC---- Iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii I J i i?__-i i . . _ Nitrogen flow

Figure 6. Assembly of detector chamber. (TEC stands for thermoelectric coolers.)

20
0

-1

-2

< -3

_-4

cO ----5

---6

-7

-8

.... 2

< -4

-8

-10

× 10-5

-5 -4 -3 -2 -1 0

Bias voltage, V

(a) D 13-K203-19# 1.

0

<

_" ---1

<9

-3

-4

-5

-6

× 10 -5

...... _-- 20

-25 -20 -15 -10 -5

Biasvoltage, V

(b) D15-K204-13#5.

-2O

× 10 .5

°C

0

20

-15 -10

Bias voltage, V

(c) D 19-K2(14-11 #3.

-5

× 10 .4

oC

20

0

20

-20 -15 -10

Bias voltage, V

(d) D19-K204-11#6.

-1

< ----2

---3

<)

-4

× 10---5

-5

-6

D20-K200-2

D19-K204-11#8

D15-K204-13#8

-10 -8 -6 -4 -2 0

Bias voltage, V

(e) Compares V'I for different samples at 22 °C.

Figure 7. Voltage-current (V-I) characteristics at different temperatures for InGaAsSb detectors.

21
.8

.6

ra .4

©

e<

.2 ..f

_'-f , ,_, ....

0 , , ,

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(a) D13-K203-19#1.

1.6

1.2

©

.4

0

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(b) D15-K204-13#8.

1.6

1.2

> .8

=

©

,4

\

0 i i n _ m i "----

1000 12(}(} 1400 1600 1800 2000 2200 2400

Wavelength, rtm

(c) D 19-K2(14-11 #6.

2.0

1.5

i_ 1.o

©

.5

J

J

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(d)D19-K204-11#8.

2o

> 1.0

©

0

1000 1250 1500 1750 2000 2250

Wavelength, nm

(e) D20-K200-2.

Figure 8. Spectral response calibration of InGaAsSb detectors obtained at 20 °C and 0-V bias voltage.

22
0

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

Figure 9. Detectors' quantum efficiency comparison at spectral calibration operating conditions.

23
1.0

.8

,<

-_ .6

_,.4

0

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(a) D13-K203-19#1.

1.6

1.2

>

9 .8

©

.4

V

0

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(b) D15-K204-13#8.

2.5

2.0

1.5

>

1.o

r_

.5

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(c) D 19-K204-11 #6.

3.0

2.5

_2.0

> 1.5

©

1.o

a<

.5

lO.5

lOOO 12oo 14oo 16oo 18oo 2000 2200 2400

Wavelength, nm

(d) D19-K204-11#8.

2_0

1.5

> 1.0

e<

.5

V

8.5

7

8

()tiler bias voltages

1, 2, 3, 4, _md 5 V

1000 1250 1500 1750 2000 2250

Wavelength, nm

(e) D20-K200-2.

Figure 10. Spectral response variation with bias voltage for InGaAsSb detectors obtained at 20 °C temperature.

24
,2[

,<

_ ,4

oC

20

10

0

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(a) DI 3-K203-19#1.

1.6

1.2

> .8

.4

20

10

0

....10

-20

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(b) D 15- K204-13#8.

1.6

1.2

> .8

=

©

,4

oC

20_..

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(c) D 19-K204-11 #6.

2.5

2.0

._ 1.5

>

o 1.0

.5

°C ,1111

20 _-...

10--.__- N

0 .... 2"_

-10 ----_-_"

-20

1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, nm

(d) D 19-K204-11#8.

2.0

1.5

<

e5

'_ 1,0

.5

oC

20

10

0

....10

....20

0

1000 1250 1500 1750 2000 2250

V_2avelength, nm

(e) D20-K200-2.

Figure 11. Spectral response variation with temperature for InGaAsSb detectors obtained at 1-V bias voltage.

25
.%! 10 -9

e:

"_ 10-1o

¢)

-d

r_

10-11

_ 10-12

_...,.I_---4- .-_--___ ___._-'_

---_lf" j DI3-K203-19#1

/ ¢'/ I _ DI 5-K204-13#5

L..-¢ ........ DI 9-K204-11#6

I + DI9-K204-11#8

t______R2__9___:___2___o___o_:__ 2_...........

0 2 4 6 8 10 12

Bias voltage, V

(a) Vadation with bias voltage at 20 °C.

14

r_ 10 -9

<

'_ 10-10

¢)

.-d

10_11

¢9

Z 10 -12

.......... -....

._____,____________------ ....

.--e---D13-K203-19#1

D15-K204--13#8

_- D19-K204--11#6

--+- D19-K204--11#8

_..__..___p_Z o._:.g_.2__.o._ o_::2._.................................

-40 -30 -20 -10 0 10

'I_mperattlre, °C

(b) Variation with temperature at bias voltages

specified in text.

2O

Figure 12. Variation of noise current spectral density.

10-9

r-_! lo-lO

z lO-11

lO-12

9#1

#8

DI 9-K204-11 #6

I

10 lo

109

108

_D19-K204-11#6

1000 1200 1400 1600 1800 2000 2200 1000 1200 1400 1600 1800 2000 2200

Wavelength, nm Wavelength, nm

(a) NEe. (b) D*.

Figure 13. Noise-equivalent power (NEP) and detectivity (D*) variation with wavelength at spectral calibration

operating conditions.

............. ..........................................

!

!

Figure 14. Signal-to-noise ratio (SNR) measurement using optically chopped signal.

26
101-2o

Temperature,°C 'I_mperamre,°C

-10 0 10 20 -20 -10 0 10 20

1.2

;a

z

©

100

10-1

10---2

o []

[]

[] [3

[3

[3

[3

O

O

O

o

SNR variation wilh

[] Bias voltage

o Temperature

1

[]

O

_o

O

[]

[3

_%

1.0

.8

z

2 3 4 5 6 7 8 9

Bias voltage V

(a) D20-K200-2. (b) D19-K204-11#8.

SNR variation with

[3 Bias voltage

__._o_ ........ j___.p___::a__a___._ ...........

[]

O

0 0

0 0

[]

D [] [30 0

2 4 6 8 10

Bias voltage, V

Figure 15. Signal-to-noise ratio (SNR) measurements for InGaAsSb detectors. SNR variation with temperature

obtained at 1-V bias and SNR variation with voltage obtained at 20 °C.

27
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0l- 02 - 2003 Technical Publication

4. TITLE AND SUBTITLE

InGaAsSb Detectors' Characterization for 2-gm CO2 Lidar/DIAL

Applications

6. AUTHOR(S)

Refaat, Tamer F.; Abedin, M. Nurul; Koch, Grady J.; and Singh,

Upendra N.

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

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National Aeronautics and Space Administration

Washington, DC 20546-0001

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755-09-00-07

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REPORT NUMBER

L-18243

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NASA

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NASA/TP-2003-212140

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Refaat, Science and Technology Corp. Abedin, Koch, and Singh, Langley Research Center.

An electronic version can be found at http://techreports.larc.nasa.gov/ltrs/or http://techreports.larc.nasa.gov/cgi-bin/NTRS

14. ABSTRACT

Recent interest in monitoring atmospheric C02 focuses attention on infrared remote sensing using the 2-gm lidar/differential absorption lidax

(DILL) technique. Quantum detectors are critical components in this technique, and many research efforts concentrate on developing such

devices for the 2-_tm wavelength. Characterization results of lnGa_AsSb quantum detectors for the 2-_tm wavelength range are presented,

including experimental setup and procedure. Detectors are prototype devices manufactured by using separate absorption and multiplication

(SAM) structures. Characterization experiments include V-I measurements, spectral response and its variation with bias voltage and

temperature, noise measurements, noise-equivalent-power (NEP) and detectivity calculations, and signal-to-noise ratio (SNR) estimation. A

slight increase in the output signal occurred with increased bias voltage and was associated with a noise level increase. Cooling down the

detectors reduces noise and shifts the cutoff wavelength to shorter values. Further improvement in the design and manufacturing process, by

increasing the device gain and lowering its noise level, is necessaryto meet the required CO2 lidax/DilL specifications.

15. SUBJECT TERMS

Infrared detectors; 2-gm avalanche photodiodes (APDs); InGaAsSb separate absorption and multiplication (SAM) structures;

Noise-equivalent power (NEP); Detectivity (D*); Signal-to-noise ratio (SNR)

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