Abstract:
An apparatus (10) is provided for measuring the excess carrier lifetime in a semiconductor material, such as an HgCdTe wafer (MCT). The apparatus includes a computer controller (56) which automates the functions of the apparatus, including the operation of the shutter (28) to control the time the testing samples are exposed to the excitation energy from a laser (14), the laser energy intensity on the sample, the position of the wafer controlled by the computer controller operating a motorized sample positioner (39) and maintaining the temperature of the sample. Multiple samples are taken by the apparatus which are averaged and analyzed to result in a characterization of the carrier lifetime.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to an apparatus for measuring the excess carrier lifetime in a semiconductor, specifically HgCdTe wafers. 
     BACKGROUND OF THE INVENTION 
     Measurement of the excess carrier lifetime is extremely important in characterizing and prescreening semiconductor materials. In the past, this measurement was often done by a contact technique, attaching electrodes directly to the semiconductor material being tested. A much better technique has more recently been used with noncontact lifetime characterization apparatus. 
     The current noncontact characterization apparatus in use are very large and complex devices. It takes a skilled operator to operate them correctly and they are slow and cumbersome in operation. A need therefore exists for an improved apparatus for measuring excess carrier lifetime characteristics which overcome the complexity and slowness of the prior designs. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, an apparatus is provided for measurement of excess carrier lifetime of a semiconductor material. The apparatus includes a first source of energy to excite the excess carriers in the semiconductor material. A second source of energy is used to measure the conductance of the semiconductor material. A computer controller is provided which functions to automatically expose the semiconductor material to the first source of energy to cause a predetermined quantity of energy to be transferred to the semiconductor. The computer controller further functions to monitor the change in conductance of the semiconductor material, analyze the change, and calculate the excess carrier lifetime of the semiconductor material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further advantages thereof, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic view of an apparatus forming the first embodiment of the present invention; 
     FIG. 2 is a side view of the apparatus; 
     FIG. 3 is a schematic view of the apparatus illustrating the electronics used; 
     FIG. 4 is a schematic of the detector bias circuit; 
     FIG. 5 is a representative data graph showing data collected and analyzed by the apparatus; 
     FIG. 6 is a representative log data graph showing the data; and 
     FIG. 7 is an exploded view of a four sample holder. 
    
    
     DETAILED DESCRIPTION 
     With reference now to the accompanying figures, wherein like reference numerals designate like corresponding parts throughout the several views, there is shown in FIGS. 1 and 2 an apparatus forming a first embodiment of the present invention for measurement of the excess carrier lifetime of a semiconductor material 12. The material 12 being tested is commonly a HgCdTe (MCT) wafer and will be referred to as MCT sample 12. The apparatus is intended to measure lifetimes from less than 10 nanoseconds such as found in high doped (greater than 10 16  electrons per cubic centimeter) p-type material to greater than one microsecond for low doped (less than 10 15  electrons per cubic centimeter) n-type material. 
     A laser 14 forms a pump source to energize the excess carriers in the MCT sample 12. The laser 14 is preferably a YAG, YLF or diode laser. The beam 16 from the laser passes through beam conditioning optics 18 and to an optical E-O (electro-optic) modulator 20. The modulator 20 controls the duration of time that the MCT sample 12 is exposed to the beam 16. From the modulator 20, the beam 16 passes through a removable turning mirror 22 and to a beam splitter 24. The beam splitter directs part of the beam to a Si (silicon) detector 26 which monitors the Temporal Profile of the beam. The remainder of the beam passes through a shutter 28, an attenuator 30 and to a second beam splitter 32. The beam splitter 32 directs a portion of the beam to a Si (silicon) energy probe 34. The remainder of the beam passes to a turning mirror 36 and is directed to the semiconductor material sample 12 positioned within a test Dewar envelope 38 which maintains the sample at a predetermined temperature, typically between 80°-300° K. with liquid N 2 , although colder temperatures can be achieved by using liquid helium. The test Dewar envelope 38 rests on a X, Y, Z axis or three axis motorized sample positioner 39. Positioner 39 allows the envelope 38, and sample 12, to be moved along three perpendicular axes to position the sample 12. 
     A millimeter (MM) wave source 40 generates energy of a certain wavelength, preferably a wavelength of 3.3 millimeters, having frequency of 90 megahertz. The energy from source 40 is directed along a millimeter wave guide 42 to a millimeter lens horn 44. The lens horn 44 focuses the energy from the source 40 onto a predetermined area of the semiconductor material 12. 
     Some of the energy directed to the semiconductor material 12 by source 40 will be reflected back to the lens horn 44. This reflected energy travels along the wave guide 42 and is collected by a millimeter (MM) wave detector assembly 46. As the energy from the laser 14 is absorbed by the semiconductor material 12, the excess carriers will be excited, which will change the conductance of the material 12. This conductance change results in a variation in the energy reflected back to the lens horn 44 which is then detected and analyzed to result in a measurement of the excess carrier lifetime as will be described hereinafter. 
     FIGS. 2, 3 and 4 illustrate the processing of the data contained in the reflected millimeter wave energy. The millimeter wave energy is provided to a millimeter Schottky detector 48 and through a detector bias circuit 50 to an FET probe 52. The signal from the probe is provided to a digitizer 54 which digitizes the analog data measured by the FET probe 52. This digitized data is provided to a computer controller 56 which analyzes the data and computes the excess carrier lifetime of the semiconductor material. As can be seen, the computer controller also controls the operation of the shutter 28 through a shutter driver 58 and of the positioner 39 to position the MCT sample in the X, Y, and Z directions. The computer controller also controls the temperature of the MCT sample and the dewar envelope 38 through a temperature controller 41. The computer controller can also control the E-O modulator 20 and silicon energy probe 34. The computer controller also controls the laser current and pulse repetition rate of the laser 14 by input of the laser pulse energy from the silicon energy probe. The computer controller also controls when the MM wave source 40 and MM wave detector are turned on and their bias voltage. 
     The apparatus provides a compact, fully contained measurement instrument offering complete computer control. The compact apparatus could be contained in an installation with a footprint less than three foot by four foot with all measurement equipment housed in two stand alone racks 59 and 61 as seen in FIG. 2. The apparatus offers computer control of the entire test analysis procedure. Thus, specially trained operators would be unnecessary. 
     If desired, contact type excess carrier lifetime studies could be done as well. In addition, the test can be made all optical with a YAG laser used as a pump to excite the material and a CO 2  laser used as a probe to detect the change in transmittance of the MCT sample. The test can be photoconductive with a YAG or CO 2  laser used as a pump to excite the material and leads attached to the MCT sample to detect the change in resistance of the sample. The YAG laser would have a wavelength of about 1.06 micrometers, while the CO 2  laser would be tunable from about 9-11 micrometers wavelength. A short pulse GaAs diode laser can also be used. 
     In one apparatus constructed in accordance with the teachings of the present invention, laser 14 is a Quantronix 113 CW/Q switched laser having a wavelength of 1.06 micrometers. The positioner 39 is an Aerotech Unidex 11 Micropositioner. The millimeter wave source 40 is a Hughes 47136H Impatt diode source of 50 milliwatts. The Dewar envelope is a Janus model R/D liquid nitrogen Dewar, or if liquid helium is used, a model ST. The temperature controller is a Lakeshore cyrotronics model DRC/81C. The lens horn 44 is an Alpha 857W horn lens antenna. The Schottky assembly 46 is a Hughes 47326H Schottky diode detector. The computer controller is a personal computer based on a 486 microprocessor which is operated by a basic program which is listed beginning on page 12 of this application. 
     The FET probe 52 is a Tektronix P6201 FET probe which is connected to a Tektronix 7912AD digitizer having a Tektronix 7A24 plug in. 
     With reference to FIGS. 5 and 6, the analysis of the data can be understood. FIG. 5 is a direct plot of the signal variation in the millimeter wave detector assembly 46 which shows the signal rises quickly to a peak value 60 while the MCT sample 12 is exposed to the energy of the laser 14, and decays thereafter as the excited excess carriers return to their normal state. The Y axis is signal voltage above a constant DC value representing the steady state reflected millimeter wave energy. The X axis is time, extending about 5000 nanoseconds. FIG. 6 illustrates the same data on a log plot versus time which, ideally, should illustrate a linear decay. 
     The controller 56 provides the capability to average multiple waveforms, such as shown in FIG. 5, correct for baseline noise, calculate the carrier lifetime, and tabulate, plot and store the results. 
     As proposed herein, the computer controller 56 would also provide control of the energy delivered to the test sample. The computer would control the position of an attenuator (set of optic filters or prisms) such as attenuator 30 in the laser beam. The laser energy on the test sample could be automatically adjusted by varying the rotational position of this optic. 
     Among the advantages of the computer controller 56 is control of the mechanical shutter 28 to decrease test cycle time and reduce chances for operator error. 
     Temporary storage of individual waveform files is made in the computer&#39;s RAM prior to averaging and analysis. This method increases test speed and conserves hard disk storage space since only the final average baseline corrected, normalized waveform is stored on the hard disk. The computer provides multiple waveform averaging, baseline noise correction, waveform polarity correction, normalization, decay function log calculation, and least squares linear regression fit of the decay function to calculate carrier lifetime. The computer also provides automatic storage of only the final normalized waveforms on the hard disk. An automatic display of the normalized waveform is provided on CRT monitor 70 immediately following the test which provides a quick feedback on how the setup is functioning. The program also provides an operator-selectable CRT display of log waveform and regression fit. Thus, the operator need not wait for a printed curve from printer 72 to review the results and the data is available for inspection immediately following each test. 
     When using a YAG laser 14, the duration of the light incident on the sample is expected to be in the range of about 2-40 nanoseconds. When using a CO 2  laser 14, the duration would be in the range of 10 nanoseconds to 10 milliseconds. 
     Use of the sample positioner 39 will permit the computer controller to inspect multiple spots on the MCT sample 12. For example, a center sample can be taken, followed by samples from a spot one millimeter above the center location and one millimeter below the center location. Also, a sample one millimeter to the left of the center position and one millimeter to the right of the center position can be taken. 
     As seen in FIG. 7, a multiple sample container 62 can be mounted in the Dewar envelope which has four apertures 64 therein, each aperture receiving a particular MCT sample for measurement. Thus, four different samples can be examined without disturbing the setup of the Dewar envelope. The container 62 includes a Dewar cold pedestal 100, a base plate 102, a clamp plate 104 and a cold shield 106, all held together by screws 108. The cold pedestal 100, base plate 102 and clamp plate 104 are preferably made of Copper. 
     In samples taken with an apparatus constructed in accordance with the teachings of the present invention, the spot size of the laser beam 16 on the sample beam tested was typically 4 millimeters in diameter. Irradiance was maintained in a range from 1 to 400 watts per square centimeter for the duration of the incidence, typically 10 nanoseconds. Sixty-four waveforms were averaged by the digitizer 54, having been taken over a period of approximately six seconds with a sample rate of about ten hertz. 
     The present apparatus can be used to determine lifetime characteristics for both short lifetime, high doped P-type MCT material and long lifetime, low doped N-type MCT material. The materials can be probed with both front side and back size (i.e., through the substrate) illumination. 
     The 7912AD digitizer digitizes up to ten waveforms per second at up to two gigasamples per second. The data acquisition sequence is controlled by the computer controller 56 over a GPIB (General Purpose Interface Bus, IEEE Standard 488). Digitization of 512 points in each waveform is initiated by a trigger derived from the laser output. A cumulative average of sixty-four waveforms is taken with the shutter open, from which is subtracted the cumulative average of sixty-four waveforms with the shutter closed. This procedure serves to remove systematic DC offset and EMI coherent with the laser pulse frequency. The cycle is repeated several times with data being transferred from digitizer to computer controller memory at the end of each cycle for digital cumulative averaging to reduce random noise. 
     Although the present invention has been described with respect to a specific preferred embodiment thereof, various changes and modifications may be suggested to one skilled in the art and it is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims. ##SPC1##