Fir photoconductivity in epitaxial InP

Infrared Phys. Vol. 23, No. 6, pp. 311-319, Printed in Great Britain

0020-0891/83 $3.00 + 0.00 Pergamon Press Ltd

1983

FIR PHOTOCONDUCTIVITY

IN EPITAXIAL

InP

K. M. LAU* and W. L. WILSON JR Department of Electrical Engineering, Rice University, Houston, TX 77001, U.S.A. (Received 17 f&v 1983) Abstract-FIR photoconductivity from shallow donors in n-type InP was studied using Fourier transform spectroscopy. The Is+2p transition frequency was found to be 45.5 4 0.2cm-’ in zero magnetic held. Spectral response measurements made in magnetic fields from 0 to 40 kG reveal Zeeman transitions of the Is+2p (m =O, +l) states. Laser ma~etos~troscopy was also investigated, employing an optically-pumped FIR laser. There is excellent agreement between the results obtained by the two experiments. From the transition energies, an effective mass of 0.077 * 0.003 m, is obtained. With the experimentally-determined ls++2p transition energy and the effective mass, the static dielectric constant of InP at 4.2 IS was found to be I I .8 k 0.2.

INTRODUCTION

Submillimeter wave detection is important in many research areas such as diagnostics of plasmas, astronomical observations and material spectroscopy. There is a growing need, throughout the FIR frequency range, for sensitive and fast-response detectors. One of the cooled detectors, the GaAs extrinsic photoconductor, which has a maximum responsivity near 280 pm, has been studied extensively in the past decade. The most complete study has been done by G. E. Stillman and colleagues at Lincoln Laboratory. They first observed FIR photoconductivity in n-type GaAs with DCN and HCN lasers.“) With high-purity epitaxial GaAs (donor concentration in the mid lOi crns3 range), they obtained the photoconductivity spectrum and identified the peaks as ground state to excited states transitions. (‘I It was found that the experimental results match extremely well with those calculated from the hydrogenic model. Based on the same principle of hydrogenic-model effective-mass theory, another III-V compound, InP, with slightly different effective mass and dielectric constant, has a photoconductive peak response at a shorter wavelength than GaAs, near 220 pm. Of all III-V compound semiconductor materials, GaAs has been the most widely used for microwave devices such as Gunn diodes, FETs and avalanche diodes. As the requirement of these devices moves toward the millimeter wave region, GaAs is limited by the electron-saturated velocity at high electric field. InP has a higher saturated drift velocity than GaAs, and hence is expected to make more efficient devices at high frequencies. In order to prepare even better material for high-quality devices, more understanding of the characteristics of the active epitaxiaf layer is required. FIR photoconductivity is one of the techniques which can be used to identify impurity species and their concentrations. In this paper, we report a study of the FIR photoconductivity of InP. Results are compared with the theoretical model. EXPERIMENTS

The detector materials used for our experiments are epitaxial layers grown at two laboratories with different methods and reactor systems. Samples provided by the compound semiconductor facilities at Cornell University are high-purity InP layers grown by liquid-phase epitaxial (LPE) techniques on Cr- or Fe-doped semi-insulating substrates. These epilayers were not intentionally doped during growth. Since electrically-significant amounts of Si existed in both the source In and

*Present address: Department of Electrical and Computer Enginee~ng, University of Massachusetts, Amherst, MA 01003. U.S.A. 311

312

K. M.

LAU and W. L. WILSON JR

InP and since Si has a high distribution coefficient at the epitaxial growth temperatures, it is probable that Si is the major donor in this material. This was later confirmed by SIMS (secondary ion mass spectroscopy) experiments.“’ Long hours of prebaking could remove much of the Si in the melt and subsequently grown layers had free-carrier concentrations ranging from high 10IJ to low lOI cmp3 with liquid-N1 mobility, p,,, ranging from 8000 to 94,000 cm’ V ~-’ set ‘. Samples supplied by the Central Research Laboratory of Varian Associates were vapor-phase epitaxial (VPE) layers grown by the PCl,/In/H, techniques. (4)S was used as a shallow donor by introducing H,S vapor into the reactor system. Electrical contacts to all samples were made by alloying tin dots onto the epilayers in a pure Hz atmosphere. Fourier transform spectroscopy was employed to study the extrinsic photoconductivity arising from electronic transitions in the shallow donor states of InP. When InP is cooled to 4 K, most of the conducting electrons are “frozen-out” and bound to to the ground state of donor atoms. Experimentally, this is indicated by a dramatic increase in the resistance of the sample material as the temperature is lowered. FIR photoconductivity in InP is a two-step process. When submillimeter radiation is applied to the sample, electrons in the ground states are excited to the higher-bound states by absorption of the incident photons. The subsequent transition of the electrons to the conduction band is completed by absorption of phonons. The Is- +2p transition has the highest absorption coefficient among other excited-state transitions because of its higher oscillator strength. This was first seen in the photoconductive spectrum of GaAs.“’ The two-step ionization process is supported by the measurements of the resistance and relative photoresponse of InP as a function of temperature. (j) As temperature is decreased to about 7 K, the resistance saturates, which indicates the freeze-out of all conducting electrons. The responsivity also peaks at about the same temperature. When the temperature is lowered further, the responsivity decreases because there are not enough phonons to complete the ionization and some of the bound electrons in the 2p level are recaptured in the ground state, from where ionization is improbable. A commercial Fourier transform spectrometer with a radiation source chopped at a frequency of 15 Hz was used. The photoconductive signal was measured as a function of the optical-path difference. A fast Fourier transform (FFT) routine was used to calculate the photoconductive signal as a function of wavenumber. The setup for interferogram recording is shown in Fig. 1. The movable mirror of the interferometer is driven by a SLO-SYN Preset Indexer which can step the mirror position by a minimum of 2.5 pm. For experiments with no magnetic field, a 30 pm step size was used most of the time. This corresponds to an optical-path difference of 60 pm and a maximum wavenumber of 83.3 cm--‘. FIR radiation was transmitted by a light-pipe system consisting of i” copper and brass pipes with polished inside walls, and a thin-wall stainless-steel (SS) pipe for the cooled section.

PUNCH STEP COMMAND

FOURIER TRANSFORM SPECTROMETER

15Hz

CONTROL CIRCUIT

CONVERT

COMMAND COMMAND

REFERENCE LOCK-IN AMPLIFIER

A/D CONVERTER

PAPER TAPE PUNCH

I nP DETECTOR Fig.

I.

Experimental

arrangement

for intcr~wogram

rccordmg

-

FIR photoconductivity

of InP

313

The detector was mounted on a Teflon holder attached to the end of the SS light pipe, which was then immersed in a liquid-He storage Dewar flask. Since most samples had areas much smaller than the light-pipe cross section, a 10” brass cone was used to condense radiation onto the detector. A black polyethylene filter located at the top of the pipe was used to filter visible and near-infrared radiation. The bias circuit includes a set of switchable load resistors at room temperature. Load resistance was chosen to be close to the detector 4 K resistance and the device was biased just below impact ionization breakdown which then achieved a maximum signal-to-noise ratio. Detector response was measured with a lock-in amplifier. The analog signal from the amplifier was digitized by an A/D converter and then recorded on paper tapes for subsequent input into a computer. The automatically-recorded interferogram was fed to a PDP 1 l/45 minicomputer for analysis. Complete frequency spectra were also taken at constant magnetic fields with the Fourier transform spectrometer. The experimental arrangement was similar to that described above. Instead of using the storage Dewar, the InP detector attached to the end of a light pipe was placed at the center of a superconducting magnet, which provides a magnetic field of up to 50 kG for the Zeeman effect experiments. In addition to the Fourier spectrometer, a FIR laser was also used as a submillimeter wave source for this photoconductivity study. Laser lines with milliwatt power levels and precise frequencies are readily obtainable. The submillimeter laser consisted of an optically-pumped system built in-house. A 1.1-m long, grating-tuned CO, laser which radiates 10-40 W in all four branches was used to optically pump a variety of lasing gas in a 2-m long resonant cavity. Strong and stable laser lines frequently used are at wavelengths of 103, 119, 133, 148, 163 and 184pm. In the magnetic field experiments, the FIR laser was employed as a high-power fixed-frequency source, and the magnetic field was scanned to generate a magnetic response spectrum for several samples. The experimental setup is shown in Fig. 2. A square-wave chopper was used to modulate the laser. The wavelength of the laser was determined by directing the laser output through the Fabry-Perot interferometer to a GaAs detector in the storage Dewar.

/

-I

CO, LASER

1

FIR LASER FABRY - PEROT

h

1

-Gzb DETECTOR Y

X

RECORDER

IflP DETECTOR

SUPERCONDUCTING HALL

Fig. 2. Arrangement

MAGNET

GENERATOR

for photoconductivity

measurements

with magnetic

field.

K. M.

314

LAU

and W. L.

WILSON

JR

A particular FIR laser line was used to illuminate the InP test sample. The magnetic field was swept up or down, and the photoconductive response of the detector was measured as a function of the magnetic field. EXPERIMENTAL

Spectral

response

RESULTS

at zero magnetic jield

Some representative photoconductivity spectra are shown in Fig. 3. The dominant peak (A) corresponds to the ls+2p transition and is very distinct in every spectrum. The exact frequencies of the peaks are listed in Table 1. The theoretical resolution of the spectra, which approximately equals the inverse of the maximum optical-path difference, is 0.65 cm -I. However, variation of the peak position is less than 0.3 cm -’ for repeated experimental runs on the same sample. From the spectra and Table 1, it can be seen that the 1s-‘2p peak position varies from the lowest value of 44.9 * 0.2 cm-’ to the highest of 45.8 + 0.2 cm-’ among different samples. The possible origin of the variation will be discussed below. The energy of the ls+2p transition is given by m* 1 __ AE li *zp-?R -4 ’ m, E:’

II

I 45

40

I 50

I 50

(cm-’

WAVENUMBER

I

Fig. 3. Spectral

response

1

I 50

I 45

40

(1)

WAVENUMBER

(cm-‘)

of InP samples

at zero magnetic

I 55

field.

FIR photoconductivity

Table Sample NO. NRL132S NRL134S Wl5lS Wl55S Wl58S Wl87S Wl9lS NRL204S NRL205S SB59 ssw52- 1 sswss-2 SSW54-2 ssw59- I

I. Transition-energy

315

of InP

measurements

1~42~ Transition peak position (cm- ‘) 45.4 45.7 45.5 45.2 44.9 45.5 45.4 44.9 45.8 45.1 45.0 45.5 45.8 46

in zero magnetic

field

Is--+3p Transition peak posltion (cm

‘)

48.5-50.5 (broad) 52.2 51.8 52. I 51.8 50.9 (broad) 52.2 51 50.5 51 (broad) 49 (broad) 50 (broad) 51.3

With the experimentally-determined frequency of 45.5 f 0.2 cm-‘, which is the most common value observed, and the room-temperature static dielectric constant of 12.35,(h) the effective mass ratio of InP is found to be 0.084 f 0.001. It is also seen in most spectra that there are two distinctive secondary peaks near 48 cm ’ (B) and 52 cm-’ (C). The energy of the ls-t3p transition calculated from the observed value of the ls+2p transition is found to be 53.9 cm -I. Thus, the one near 52 cm ’ is identified as the ls-t3p transition. On some other samples, these two peaks merge into one broad peak centered around 50 cm-‘. The shifting and broadening of this higher-energy transition should not be surprising since this peak was only observed in ultra-high-purity GaAs with a donor concentration less than the low 1014cm-3.(7) Broadening of excited states in shallow donors Since the variation of the 1~42~ transition energy observed seems to be greater than experimental uncertainty, some explanation is required. The theoretical calculation of assumed broadening of impurity bands,(*) may provide an answer. In this model, Baltensperger the impurity atoms are regularly spaced and found the radius of a Wigner-Seitz sphere from impurity concentration. The radius rs is given by 47L

1

-$_

3

the the that the

’

N,,’

(2)

He solved the Schrodinger equation for an electron in the sphere subjected to Coulomb potential and imposed the boundary condition that the wave functions be in Block form. The edges of the Is, 2s and 2p bands were determined by the boundary conditions. The results are shown in Fig. 4. The energy is plotted against the ratio of r, to the Bohr radius ug. As shown in the figure, the broadening of the 2p state begins at rs/aB - 12. For InP, which has a uB of 80 A, this corresponds to a donor concentration of N,, - 3 x lOI cm p3. The 1s and 2p bands overlap for r,/ug- 2, which correspond to Nn - 6.3 x 10’6cm-3. These results suggest that our samples, with donor concentrations between the above values, have somewhat broadened 2p states which might account for the shifting of the ls+2p transition. However, the experimentally-observed shifts do not seem to have any correlation with the measured carrier concentration. Magnetic jield effects Under the influence of a magnetic field, the ls+2p transition splits into three components corresponding to the ls+2p (m = 0, f 1) transitions, as predicted by the hydrogenic model. The ls+2p, m = + 1 transition remains dominant at all magnetic fields, while the ls+2p, m = 0 transition is less distinct at low fields. The transition energies at various magnetic fields are shown in Fig. 5. With the experimentally-determined splitting of the m = + 1 transitions and the Zeeman-splitting expression, AE, I = E(2p, m = 1) - E(2p, m = _ 1) = gC

316

K. M. LAU and

0

2

4

6

W. L. WILSON JR

IO

8

12

14

16

16

i ‘“B Fig.

4. Broadening

of the

Is, 2s and

2p donor

levels

with

increasing

donor

concentration.‘H’

the effective mass ratio m */m. was deduced by a least-squares straight-line fit of the data and found to be 0.077 + 0.003, which is about 8% smaller than the value determined from the energy of the ls+2p transition in zero magnetic field. In the zero-field calculation, the room-temperature dielectric constant of 12.35 was used since there is no accurate low-temperature dielectric constant data. In GaAs, it was found that the static dielectric constant has a linear temperature dependence represented by s,(T) = s,(O) (1 + (x7-L

(4)

where s,(O) = 12.79 and a = 1.0 x lo- 4.(9)The value at 4.2 K is lower than the room-temperature value. Approximating the same temperature dependence for InP, the dielectric constant at 4.2 K was found to be 11.95 and resulted in a lower effective mass ratio of 0.079. With the FIR laser as a fixed-frequency source, we were able to obtain strong resonant transition spectra as a function of magnetic field. Within the magnetic field range of the superconducting solenoid (O-50 kG), the 1s+2p, m = + 1 transition energy extends from 45 to almost 100 cm ’ and there are five easily obtainable and stable laser lines which are well separated in this energy range. Figures 6 and 7 are the recorder traces of the photoconductive response as a function of magnetic field when the InP detector was illuminated with the 119 and 162 pm laser lines. The two traces in Fig. 6 correspond to the magnetic field being scanned from above or below the resonant transition in order to assure there was no hysteresis in the recording process. The ls+2p, m = + 1 100

20 MAGNETIC Fig,

5. Splitting

of the

ls+2p

transition

25

30 FIELD (kG)

as a function

of magnetic

field.

FIR photoconductivity

of InP

317

I

I

I

I

5

IO

15

20

MAGNETIC

Fig. 6. ls+2p,

m = +l

FIELD

transition

(kG1 at 162~1~

transition energies obtained with the laser source are also shown in Fig. 5. It can be seen that there is excellent agreement between the laser and Fourier spectrometer results. As shown in the Fourier spectra, the ls~2p, m = 0 transition energy does not increase as significantly with magnetic field, this transition was only observed with the 184 pm line (54.3 cm-‘) at 37.5 kG. There are some transitions on the low magnetic field side of the dominant lss+2p, m = + 1 transition. Their

I

I

I

30

35

40

MAGNETIC

Fig. 7. ls-t2p,

FIELD

m = +1 transition

(kG) at 119Mm

318

K. M. LAU and W. L.

I

I

20

25

MAGNETIC

Fig. 8. Is-3p

FIELD

WILSON JR

(kG)

transitions compared with ls+2p,

m = +l

transition

at 119pm.

amplitudes are four or five times smaller and usually much broader. These transitions are shown in Fig. 8, along with the ls+2p, m = + 1 transition, as a function of magnetic field. Some of these peaks can be connected with smooth lines similar to those observed in GaAs and identified as ls-+3p, m = * 1 transitions. (‘O) Their identification cannot be confirmed since neither the theoretical calculation of the appropriate energies is available nor Fourier spectra showing corresponding transitions. It is not suprising that these very weak transitions can be observed with the laser but not with the Fourier spectrometer. One obvious reason is that at a certain magnetic field, the laser energy coincides with the weak transition energy only, while with the spectrometer, the weak transitions have to compete with the strong and dominant ls-+2p transitions and might not be able to show on the Fourier spectra.

CONCLUSION The study of FIR photoconductivity in InP confirms the validity of the hydrogenic-model effective-mass theory for shallow donors in III-V compounds. The ls+2p transition is found to be at 45.5 f 0.2 cm -I, The Zeeman splitting of this transition in a magnetic field yields an effective mass value of 0.077 i 0.003 m,. This value is more accurate than the 0.084 m, value which would be determined from the ls+2p transition energy and the room temperature dielectric constant of 12.35. With the Zeeman-splitting-determined effective mass and the measured ls++2p transition energy, the 4.2 K static dielectric constant is found to be 11.8 f 0.2. There was no significant difference between the results obtained with the LPE and VPE grown samples. Shifting of the ls+2p transition peak was observed in both kinds of material and is not related to the method of epitaxial growth. Since Si and S are major donors in the LPE and VPE samples, respectively, these results suggest that the difference between the ground-state energies of these donors in InP is minimal. However, definite observations of central cell corrections in shallow donors were reported in GaAs. The lack of similar hyperfine separations (of the order of 0.5 cm-’ in GaAs) in the complete frequency spectra of InP might be due to the limitation of the resolution of the spectra, which is 0.65 cm-’ in our experiments. Using the FIR laser, however, we should be able to achieve extreme resolution in magnetospectro~opy (< 0.05 cm-‘), but still no fine structure was observed. One possible reason is that in each of the InP samples there is just one dominant dopant and comparatively small amounts of any others.

FIR photoconductivity

of InP

319

In summary, we have shown that InP can be made a useful detector in the 200 pm spectral region. With a magnetic field up to 50 kG, the detector is tunable from below 100 pm to above 250 pm. New measurements of the effective mass and static dielectric constant of InP at liquid-He temperature were obtained. Confirmation of the hydrogenic model encourages further investigations of the feasibility of utilizing other binary or ternary III-V compounds as photodetectors to cover the FIR spectrum. REFERENCES P. E. and Dimmock J. O., Appl. Phys. Left. 1. Stillman G. E., Wolfe C. M., Melngailis I., Parker C. D., Tannenwald 13, 83 (1968). 2. Wolfe C. M. and Stillman G. E., in Gallium Arsenide and Related Compounds; Proc. 3rd Int. Symp., Aachen, F.R.G., 1970. Institute of Physics/Physics Society, London (1971). 3. Ip K. T., Ph.D. Thesis, Cornell University, Ithaca, N.Y. (1978). 4. Fairman R. D., Omori M. and Frank F. B., GaNium Arsenide and Related Compounds; Proc. 6/h Int. Symp., 1976, p. 45. Inst. Phys. Conf. Ser. No. 33b. 5. Wilson W. L. and Epton P. J., Infrared Phys. 18, 669 (1978). 6. Hilsum C., Fray S. and Smith C., Solid St. Commun. 7, 1057 (1969). 7. Stillman G. E., Wolfe C. M. and Dimmock J. O., Donor Magnetospectroscopy in High Purity Epitaxial GaAs, p. 265; Proc. 3rd Int. Conf. Photoconductivity, Stanford, Calif., 1969 (1971). 8. Baltensperger W., Phil. Msg. 44, 1355 (1953). 9. Lu T., Glover G. H. and Champlin K. S., Appl. Phys. Left. 13(12), 404 (1968). 10. Narita S. and Miyao M., Solid Sf. Commun. 9, 2161 (1971).