# SCLC Degradation in 980 nm Pump Laser by  Using Electrical Noise

Authors: B. Orsal; I. Asaad
Journal: ISRN Optics
 (2012)

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## Abstract

The knowledge of the noise levels is important for pump laser diodes as it allows to study and to locate the noise sources and their origin. 980 nm fresh and aged pump lasers have been characterized by using electrical noise measurements. At 10 Hz, the spectra are dominated by1/fm (1≤m≤2) noise. Current noise spectral density (CNSD) is dominated by  ILm
(1<m≤2). The trapping defect density near the n+n- and p+p- interfaces is related to pinching of the space-charge-limited current (SCLC) effect. An excess electrical noise due to longitudinal mode hopping is correlated with optical power fluctuations.


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## Body

## 1. Introduction

In this work, we present the electrical noise at low frequency for fresh pump laser diodes emitting at 980 nm (reference laser), and we study the parametric evolution and the defects generated in aged 980 nm, stressed during 400 hrs at 50°C and high current injection (500 mA) (aged laser).


## 2. Device Description

The device used in this study is a conventional ridge waveguide AlGaAs/InGaAs pump laser with a GRIN-SCH (Graded Index Separate Confinement Heterostructure) and a Single Quantum Well (SQW) (p+p-,n+n-) emitting at 980 nm. The laser has a cavity length of 1200 μm and is coated, for the 50°C aging tests, with standard low- (≅1%) and high-reflectivity (≅90%) coatings on the facets. A different coating reflectivity and aging configuration has also been studied in a previous work [1–3].


## 3. Static Characteristic of the Pump Laser Diode

From the static analysis, current laser versus externally applied voltageIL(VL), we then deduce in Figure 1 the evolution of the differential resistance Rd=dVL/dIL. The Rd(IL) characteristic recorded after 400 h of aging under a stress at 50°C and high current density injection (500 mA) was compared with the same laser diode before aging.Figure 1
Dynamic resistanceRd versus laser current IL.Before threshold, differential resistance is proportional toIL-1; above threshold current (IL>18 mA), Rd decreases to IL-1/2. This phenomenon is usually observed when the carriers are injected from high doping material to low doping material. In our structure, the carriers are injected from n+ or p+ into n- or p-.The physical phenomenon can be explained in Figure2. The p+p- and n+n- interfaces form space charge regions. The carriers are prevented to go into quantum well by the two space charge effects; then, the carrier injection is associated to space-charge-limited current effect (SCLC) [4, 5].Figure 2
Space-charge-limited current effect observed in the laser layers.Consider the relation of Mott-Gurney:(1)IL,SCLC=9εε0μ8L3V2,
where ε is the dielectric constant, ε0 the free space permitivity, L the junction thickness, and μ the carriers mobility. We then deduce the evolution of the differential resistance Rd=dVL/dIL:(2)Rd=12⋅19εε0μ/8L3⋅1IL,SCLC.The typical evolution of the electrical and optical parameters is shown in Table1.Table 1
Evolution of the electrical and optical parameters.
Laser diodeRs (for IL = 100 mA) (Series resistance)Ith(Threshold current)Popt at IL=200 mA (Optical power)η (Efficiency)Reference1 ohm17.5 mA220 mW90%Aged1 ohm17.5 mA215 mW88%We show that the static characteristics do not allow making difference between the aged laser and the reference laser. This difference can be made with electrical noise measurements.


## 4. Low-Frequency Noise Measurements

### 4.1. Noise Equivalent Circuit

In order to obtain a better analysis of the behavior of the devices studied, it is necessary to give the noise equivalent circuit of multimode laser diode (4 modes for this Pump laser) at low frequency (Figure3).Figure 3
Noise equivalent circuit of a multimode laser diode at low frequency.This circuit is obtained by deriving the rate equations including Langevin white noise sources and1/f noise sources describing in [6], and we are considering all other noise sources to be produced by the interfaces and the trapping defect density [7].WhereR is a differential diode resistance, Rsek is an additional resistance due to coupling of spontaneous emission into lasing mode for k mode, vnek is a voltage noise source due to the fluctuation of the photon population, and in is the current noise source due to the fluctuation of the electron population. The input signal is the modulation current iL1 or the modulation voltage v1 for intrinsic circuit; SV,Rs=(αIL/fτs)Rs2 is the thermal noise due to series resistance Rs;SV,SCLC is the noise source due to space-charge-limited current effect.


### 4.2. Current Noise Spectral Density (CNSD)

We make a noise analysis by Fast Fourier Transform FFT measurements and very low-noise amplifier. The Current Noise Spectral Density (CNSD)SiL represents the current intensity fluctuations at 20 ± 0, 1°C. The CNSD has been measured over a wide frequency range from 1 Hz to 1 MHz; we give in Figure 4 a typical noise spectrum: 1/fm (1≤m≤2) decreases followed by flat noise level.Figure 4
Current noise spectral density versus frequency.Figure5 exhibits the low-frequency current noise measured at 10 Hz, as a function of the laser current. We notice that the spectra are dominated by 1/f (flicker) noise due to the carrier mobility fluctuations given by TGM Kleinpenning’s model [8]; the CNSD varies as IL2:(3)SILI=αHIL2fN,
with αH being Hooge parameter, N being free carrier number, and f being frequency.Figure 5
Electrical noise spectral densitySIL at 10 Hz versus IL.In this case, we can calculateαH (Hooge parameter) at very weak current, f=10 Hz, I=1 nA, and we found [9] that(4)αH=f⋅Si,1/fI2L2qμR=8,5⋅10-4,
where
(5)N=L2qμR,L=1200 (μm) is the sample length, q=1,6·10-19 (C) is the elementary charge, μ=11000 (cm2V−1·S−1 at 300 K for AlGaAs/InGaAs) is the carrier mobility, and R=108 Ω is the dynamic resistance. The αH value is in agreement with those given by Hooge for AlGaAs/InGaAs at 300 K [10].Region II
At 10 Hz, the CNSD varies asIL3/2, it can be explained by fluctuations of diffusion current, and, it is due to the traps placed near the n+n- and p+p- interfaces [4]. Then,
(6)SIL(10Hz)=C(N,A)⋅IL3/2f=ILVjRL∑iDiτi(1+ωi2τi2),
where C(N,A) is the coefficient that depends on the section and on the number of free carriers, RL is junction resistance of n+n-andp+p- interfaces, Vj is the applied voltage of junction, Di is a statistic factor, depending on the material, and τi is the recombination time.
This noise is related to pinching of the space-charge-limited current SCLC effect superposed with the noise due to fluctuations of current in the MQW outside the space charge region [11].Region III
At 10 Hz, we observe saturation of CNSD, the CNSD as predicted by Vandamme and Ruyven, when the laser current is lower than the threshold currentIth [12], because series resistance Rs, in this region, is not neglected in comparison with differential resistance Rd [13].Regions IV and V
The CNSD increases at 10 Hz, due to the optical gain fluctuations in the active layer related to increase of number of photons around threshold [14, 15]. In this case modeling is given by
(7)Sid,1/f=2⋅Z2∑k=14∑l=24(SVnk,Vnl/RSek2)(Z+RL+Rs)2+Rs2(qαIL/fτs)+RLILVj∑i(Diτi/(1+ωi2τi2))(Z+RL+Rs)2,
where Z is the equivalent impedance of intrinsic circuit, SVnk,Vnl is the noise source giving an excess noise due to longitudinal mode hopping which is related to output power fluctuations (exchange of power between two voltage noise sources due to the fluctuation of the photon population), τi is related to trap noise generators placed in the intrinsic layer and near n+n- and p+p- interfaces which absorb many photons in the aged active layer, and the end term of (7), in region V, becomes dominant.Region V
At 10 Hz, the level of noise shows also that a proportionality of aboutSi10Hz∝I3/2 originates to the traps in the vicinity of the n-n+ and p-p+ related to SCLC effect.
The CNSD in the aged laser is higher than that in the fresh laser; it is due to active layer degradation due to decrease of recombination timeτi.


## 4.1. Noise Equivalent Circuit

In order to obtain a better analysis of the behavior of the devices studied, it is necessary to give the noise equivalent circuit of multimode laser diode (4 modes for this Pump laser) at low frequency (Figure3).Figure 3
Noise equivalent circuit of a multimode laser diode at low frequency.This circuit is obtained by deriving the rate equations including Langevin white noise sources and1/f noise sources describing in [6], and we are considering all other noise sources to be produced by the interfaces and the trapping defect density [7].WhereR is a differential diode resistance, Rsek is an additional resistance due to coupling of spontaneous emission into lasing mode for k mode, vnek is a voltage noise source due to the fluctuation of the photon population, and in is the current noise source due to the fluctuation of the electron population. The input signal is the modulation current iL1 or the modulation voltage v1 for intrinsic circuit; SV,Rs=(αIL/fτs)Rs2 is the thermal noise due to series resistance Rs;SV,SCLC is the noise source due to space-charge-limited current effect.


## 4.2. Current Noise Spectral Density (CNSD)

We make a noise analysis by Fast Fourier Transform FFT measurements and very low-noise amplifier. The Current Noise Spectral Density (CNSD)SiL represents the current intensity fluctuations at 20 ± 0, 1°C. The CNSD has been measured over a wide frequency range from 1 Hz to 1 MHz; we give in Figure 4 a typical noise spectrum: 1/fm (1≤m≤2) decreases followed by flat noise level.Figure 4
Current noise spectral density versus frequency.Figure5 exhibits the low-frequency current noise measured at 10 Hz, as a function of the laser current. We notice that the spectra are dominated by 1/f (flicker) noise due to the carrier mobility fluctuations given by TGM Kleinpenning’s model [8]; the CNSD varies as IL2:(3)SILI=αHIL2fN,
with αH being Hooge parameter, N being free carrier number, and f being frequency.Figure 5
Electrical noise spectral densitySIL at 10 Hz versus IL.In this case, we can calculateαH (Hooge parameter) at very weak current, f=10 Hz, I=1 nA, and we found [9] that(4)αH=f⋅Si,1/fI2L2qμR=8,5⋅10-4,
where
(5)N=L2qμR,L=1200 (μm) is the sample length, q=1,6·10-19 (C) is the elementary charge, μ=11000 (cm2V−1·S−1 at 300 K for AlGaAs/InGaAs) is the carrier mobility, and R=108 Ω is the dynamic resistance. The αH value is in agreement with those given by Hooge for AlGaAs/InGaAs at 300 K [10].Region II
At 10 Hz, the CNSD varies asIL3/2, it can be explained by fluctuations of diffusion current, and, it is due to the traps placed near the n+n- and p+p- interfaces [4]. Then,
(6)SIL(10Hz)=C(N,A)⋅IL3/2f=ILVjRL∑iDiτi(1+ωi2τi2),
where C(N,A) is the coefficient that depends on the section and on the number of free carriers, RL is junction resistance of n+n-andp+p- interfaces, Vj is the applied voltage of junction, Di is a statistic factor, depending on the material, and τi is the recombination time.
This noise is related to pinching of the space-charge-limited current SCLC effect superposed with the noise due to fluctuations of current in the MQW outside the space charge region [11].Region III
At 10 Hz, we observe saturation of CNSD, the CNSD as predicted by Vandamme and Ruyven, when the laser current is lower than the threshold currentIth [12], because series resistance Rs, in this region, is not neglected in comparison with differential resistance Rd [13].Regions IV and V
The CNSD increases at 10 Hz, due to the optical gain fluctuations in the active layer related to increase of number of photons around threshold [14, 15]. In this case modeling is given by
(7)Sid,1/f=2⋅Z2∑k=14∑l=24(SVnk,Vnl/RSek2)(Z+RL+Rs)2+Rs2(qαIL/fτs)+RLILVj∑i(Diτi/(1+ωi2τi2))(Z+RL+Rs)2,
where Z is the equivalent impedance of intrinsic circuit, SVnk,Vnl is the noise source giving an excess noise due to longitudinal mode hopping which is related to output power fluctuations (exchange of power between two voltage noise sources due to the fluctuation of the photon population), τi is related to trap noise generators placed in the intrinsic layer and near n+n- and p+p- interfaces which absorb many photons in the aged active layer, and the end term of (7), in region V, becomes dominant.Region V
At 10 Hz, the level of noise shows also that a proportionality of aboutSi10Hz∝I3/2 originates to the traps in the vicinity of the n-n+ and p-p+ related to SCLC effect.
The CNSD in the aged laser is higher than that in the fresh laser; it is due to active layer degradation due to decrease of recombination timeτi.


## 5. Summary

The study of the electrical noise that represents the fluctuations of current at low frequency is very significant of degradation of the active layer. The spectra are dominated by conventional1/f (flicker) noise at weak current and the CNSD at 10 Hz is dominated by I3/2. Pinching of the Space Charge shows limited current SCLC effect.The defect is associated with carrier transport controlled by then+n- and p+p- interfaces and the trapping defect density. An excess electrical noise due to longitudinal mode hopping is related with output optical fluctuations at low frequency. The CNSD in the aged laser is higher than that in the fresh laser, certainly due to the degradation of recombination time τi.



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*Source: 841695-2012-01-09.xml*