Gyulai, József

Subašiev, Vagan Kasparovič

Čalikân, G. A.

English

Gyulai József

Subašiev Vagan Kasparovič

Čalikân G. A.

University of Szeged

ON THE PHOTOCONDUCTIVITY OF GaP CRYSTALS By J. GYULAI 1 Research Group for Luminescence and Semiconductors of the Hungarian Academy of Sciences, Szeged V. K. SUBASHIEV and G. A. CHALIK.YAN Institute of Semiconductors, Academy of Sciences USSR, Leningrad (Received November 30, 1966) The anomalous photoconductivity response of very high-ohmic GaP crystals was investigated. On base of the experiments it can be concluded that the effect is connected with the different mo-bilities in the corresponding minima of the conduction band. Based on calculations it is shown that the presence of the effect is connected with very short lifetimes. Investigation of the photoconductivity response curves (PCR) of GaP brought some interesting new effects: first the anomalous photoconductivity response ( A P C ) 1 , observed firstly by D . F . NELSON, L . F . IOHNSON and M . GERSHENZON [1], and successively investigated by the authors [2], [3], and furthermore, an interesting oscillation effect, attributed to interactions between excess carriers and longitudinal optical phonons [4]. The present article gives some more details on our work con-cerning the problem of APC, the conclusions having been published in [2], [3]. Experimentál methods The GaP crystals investigated were vapour-grown whiskers (very high-ohmic, dark conductivity (7d(296°K) l O " 1 ^ " 1 cm"1 ; dimensions 0,6X0,6X0,05 cm3) and platelets (dark conductance 2d(296°K)%3• lO- 1 0 ^- 1 ; dimensions 0,8 ХОД X X0,1 cm3). The whiskers were triangular in cross-section. Contacts were soldered Sn contacts with suitable impurities. The PCR measurements were carried out at either 296°K or 77°K, with and without additional steady illumination. PCR curves were taken by two independent Xe-lamp—monochromator systems, one for d. c. measurements and an other for measurements using mechanically chopped light (chopping frequency % 1350 c/s). In d. c. measurements a Zeiss S P M 1 monochromator and A megohmmeter JUPITER, (type JM 242) were used, while under a. c. conditions the monochromatic light was produced by a 3MP—2 monochromator, and the signals in this latter case were detected by a calibrated selective amplifier system (ME3, type У2—6 amplifier, ME3, type СД—1 phase-sensitive detector coupled with an automatic potentiometer). In the experiments performed at 77°K, we have used a finger type Dewar. In both arrangements it was, of course, possible to measure the incident light intensity. 1 The APC is an effect involving a second, and sometimes big rise in PCR spectral curves far beyond the absorption edge. 2 6 J. GYULAI, V. K. SUBASHIEV AND G . A. CHALIKYAN As in d. c. measurements PCR was taken point by point, the light energy was mea-sured at every wave-length by a movable mirror rendering possible to focus the light energy onto a thermocouple. In a. c. measurements, the exciting light passed through an inclined quartz plate, which reflected a fraction of it onto a PbS-cell, coupled with a selective measuring amplifier system. The apparatus was previously calibrated against a thermocouple having been set in the place of the sample. For the additional steady illumination a stabilized tungsten lamp was used, followed by suitable filters to produce various wave-lengths: glass filters, a conti-nuouly adjustable Zeiss-interference filter, Ge or Si filters. If not mentioned otherwise, PCR curves were taken by regulating the slits of the monochromator so as to keep the incident light energy approximately con-stant over the investigated spectral range. (The incident photon flux(g) was calculated on basis of this constant energy, and changes in conductivity (A a) were related to these values, i. e. AajQ was calculated.) In a. c. measurements Aa was calculated according to the formula A a = v/V0RL [5], where v is the measured a. c. voltage, F0 the voltage applied to the crystal (in our case 750 V), RL the input resistance of the amplifier, while in d. c. measurements changes in conductance (A I ) were used instead of Ao. Experimental results In order to get new informations about APC, first of all we have systematically investigated the effect of double injection. Steady illumination of wave-lengths ranging from 2,5 ev to 0,35 ev, was employed. On Fig. 1 the effect of long wave-length illumination (Si-filter) is to be seen (temperature 296°K). At room tempera-ture strengthening the intensity of steady illumination, we generally obtained the following effect: first PCR rose both in the range of indirect and direct transitions. Then from a certain intensity, PCR in the indirect region remained almost constant, while APC gradually disappeared approaching this constant level of PCR in the region of indirect transitions. This feature of PCR curves was also found when using steady light of shorter wave-lengths (Fig. 2. red filter, Fig. 3. blue filter). This fact, i. e. the disappearing of APC with increasing intensity of steady illumination, seemed to be in contradiction with the explanation suggested in [1], according to which the level of APC ought to mean some level of saturation with increasing intensity of steady illumination. At this point there still remained two possible ways of explanation which determined the means of our further experiments. According to the first i) APR would be an effect simply correlated with the decrease of penetration depth in the region of direct transitions. The values of the absorption coefficient, k, of GaP available for us [6] did not contradict such an effect, though less in magnitude. According to the second ii) APR would be due to the fact that in the region of direct.transitions, electrons are excited into the [000] minimum of the conduction band (Fig. 4a.), where the mobility (provided a scattering by acoustic modes) is about 15 times larger than in the [100] minimum. The first idea was that if APC were simply a „geometric" effect, it could have been provoked using an inclined light ray in PCR measurements, producting thus the diminution of penetration depth even at energies corresponding to indirect ON THE PHOTOCONDUCTIVITY OF G a P CRYSTALS 2 7 transitions. Unfortunately, the refraction index of GaP (« = 3,1) is rather large so as to make this effect hardly observable. Using a light inclined by 60° to normal incidence, its deviation within the substance did not exceed 15°, corresponding to an effective diminution of penetration depth only by a factor 1,06. It was surprising and pointed at the sensitivity of the measuring circuitry that some resulting change of about 1,1 times could be observed at all. Of course, it was not possible to draw quantitative conclusions from this effect. ùd .10 - s Q Quantum j t c m s j Sample % T » 2 9 6 ° K hv(ev} Fig. 1. PCR spectral curves, g « 5-1012 quantum/s. Secondary illumination through Si filter (A>l,l / i) of various intensities (curve 1: 0 quantum/s; 2: 9-1012 quantum/s; 3: 3,5-1013 quantum/s; 4: 2-1014 quantum/s) The experiment which seemed of great importance, and at last led to the most probable interpretation of APC was the following. We have measured PCR under special conditions, namely, choosing the incident intensity of exciting light so as to produce an approximately constant near-surface density of generation. This 2 8 J. GYULAI, V. K. SUBASHIEV AND G . A. CHALIKYAN constant generation rate can be approximated by keeping Qk =const, over the whole experimental range. (The least value of Q, for large k depends on the energy reserves of the illumination system.) Condition Qk = const, ensures the uniform density of generation on the surface and, at other points of the sample, a decreasing generation density with increasing hv. 2 25 3 hv(ev) Fig. 2: PCR spectral curves. ß«;3-101 2 quantum/s. Secondary illumination through red filter (type KC-10, A^0,6//) of various intensities (curve 1: 0 quantum/s; 2: 3-1013 quantum/s; 3: 1.8-1014 quantum/s; 4: 4,8-1014 quantum/s; 5: 1-1015 quantum/s) The result of this experiment is to be seen in Fig. 5. For comparison, the PCR of sample 1/1, of relatively low resistance, which does not show APC, is also to be seen in the figure. It is also to be mentioned that at the highest employed injection rates (Q > 1015 quantum/s), a strong peak appeared in PCR at 296°K, coinciding with the beginning of indirect band gap [4]. In this case APR was less expressed (Fig. 6.). At lower levels of illumination or at reduced temperatures this peak was inobservable. ON THE PHOTOCONDUCTIVITY OF G a P CRYSTALS 2 9 Investigation of PCR at reduced temperatures also presented some interesting features. Obviously, the thermal ionization of the centres in crystals. 2/4 and 2/6 was such that at 77°K a strong impurity PCR was observed (Figs. 7., 8.). The steady illumination, even at the least intensities employed, ionized these centres and APC appeared in a more expressive form compared with room temperature measurements. Fig. 3. PCR spectral curves. Q = 1012 quantum/s. Secondary illumination through blue filter (type CC5 + C3C14, 0,38/z -=¿-=0,48/;) of various intensities (curve 1: 0 quantum/s; 2: 1,5-10" quantum/s; 3: 9-1014 quantum/s) The PCR versus incident photon flux curves taken at two characteristic wave-lengths show linear PCR at room temperature, even in the range of APC, i. e. in the region of direct transitions, though Some superlinearity was observed at 3 0 J . GYULAI, V. K. SUBASHIEV AND G. A. CHALIKYAN k=[qo,o] [1,0,0] — Fig. 4a. Energy band model for GaP. 4b. Schematic diagram for calculation of APC Fig. 5. PCR spectral curves (d. c. method) taken at uniform near-surface densities of generation ON THE PHOTOCONDUCTIVITY OF G a P CRYSTALS 3 1 Fig. 6. PCR spectral curves. £2=-1015 quantum/s. Secondary illumination through Si filter 1,1/0 of various intensities (curve 1: 0 quantum/s; 2: 7-1013 quantum/s; 3: 1,8-10" quantum/s) 34 J. GYULAI, V. K. SUBASHIEV AND G. A. CHALIKYAN № ,-2£ 10 27 A tf / Q'crn s Q ' Quantum ) Sample ^ T-77°K 2,5. hv(ev) Fig. 7. PCR spectral curves. 1014^-1015 quantum/s. Secondary illumination through Si filter ( / >1,1 /i) of various intensities (curve J : 0 quantum/s; 2: 7-1013 quantum/s; 3: 1,8-1014 quantum/s) ON THE PHOTOCONDUCTIVITY OF G a P CRYSTALS 3 3 10 zr « -28 -29 10 > A 6 i S t m s \ Q 1 Quantum Sample /̂g T-77°K H h 15 hv(ev) Fig. 8. PCR spectral curves, g «s 1013 quantum/s. Secondary illumination through Si filter (A>1,1//) of various intensities (curve 1:0 quantum/s; 2: 1-1014 quantum/s) 1 = 478 nm and at r = 7 7 ° K (Figs. 9., 10.). This superlinearity can be interpreted as in CdS [7] by the actual relative position of the Fermi level at 77°K and the re-combination centres involved. Discussion The PCR curves taken with double illumination, as pointed out, are in contra-diction with the explanation suggested in [1]. The experiment involving constant near-surface density of generation enabled us to decide between the explanations i) and ii). Namely, the existence of APC under this special condition would.be in contradiction with explanation i), while it is consistent with explanation ii). Inversely, explanation ii) does not lead to any contradiction with known experimental facts, therefore it is to be considered as the most probable explanation of APC. 3 Fig. 9. Dependence of Aa on illumination intensity (Q) in the region of indirect transitions. Second-ary illumination through glass filter (type HKC-2, A>0,95/i) of various intensities (curve 1: 0 quan-tum/s; 2: 6,3-1013 quantum/s; 3: 3-1014 quantum/s; 4: 7-10" quantum/s; curve 5: 0 quantum/s; 6: 7-1013 quantum/s; 7: 1,8-1014 quantum/s) 10 ru 10 10 Sample ^ T=296 T - . 7 7 \ = i i 3 4 n m = 10 45 10' 10' 10 q ^ Quantum) Fig. 10. Dependence of Aa on illumination intensity (Q) in the region of direct transitions. Second-ary illumination through glass filter (type HKC-2, A>0,95/i) of various intensities (curves 1 and 4: 0 quantum/s; 2 and 5: 6,6-1013 quantum/s; 3 and 6: 1,8-10" quantum/s) 3 6 J . GYULAI, V. K. SUBASHIEV AND G. A. CHALIKYAN On Fig. 4a. the band scheme of GaP is .to be seen. Using the notations of the figure and those of Fig. 4b., the conductivity change by illumination (Aa=a, —ad) can be expressed as d Ac = — qJ{¡xlAn1 + fi2n2 + npAp)dx (steady state), ' o (iq charge on the electron). A) In the region of indirect transitions (egl Shv^ksg0) we have n2 = 0, An1/x1 = = Ap/xp. Using Anjx1 = G = (Qllb)k exp ( — kx), for very low values of diffusion coefficient ^indir. = t - q j (Wi + npxp)Gdx. 0 B) For direct transitions (hvSeg0), where An1/zl + n2lx2 = Ap/xp, H2/T2 + + n2jx2 = G and n2lx3 = AnJx1, we have d , b Aoau. = j q 0 0 o r , b y Ap/x =G, n2 = Gx2x3/(x2 + x3) a n d An1/x1 = Gx2/(x2 + x3), p d iir. = j q j l ^ i ^ i 0 ¿ffd ,  n- l +Wp+ Gdx. The amount of APC can thus be calculated using the ratio A(Td; a / UoTo — UiT, T2"rT3 Gdx i r . ^ i n d i r . f l U ^ + UpXplGdx Supposing t 1 ; x2 and x3 to be independent of generation density and thus of x, we have Aa&u. = t ! W z - W i T3 _ a Adiaiiu t2 + T3 Taking a % 10 for APC, this value can be explained by e. g. p2 = 15/ix, x2^x1^x3 and HpXp-^iiiXi. This explanation is in good agreement with the fact that APC takes place only in crystals of very low PCR, i, e. of very short lifetimes. The lifetime e.g. in sample 2/4 can also be calculated approximately by the relation hv I ,rr „ r * = — -rWQRLIAn, q b ON THE PHOTOCONDUCTIVITY OF GaP CRYSTALS 3 7 where IA the absorbed light energy and ¡.i the drift mobility of majority carriers (supposing ц = 102 cm2 vol t - 1 s - 1) . Using the signal at 431 nm, the formula yielded x^5-10~ 9 s , in a very good agreement with [1]. * * * Thanks are due to Prof. A. BUDO, member of the Hungarian Academy of Sciences for rendering possible to perform some experiments in this institute, furthermore, to A. A. VOLFSON (Leningrad) for allowing to use his apparatus in a. c. measurements. One of the authors (J. Gy.) is indebted to the Hungarian Academy of Sciences for a scholarship. References [1] Nelson, D. F., L. F. Johnson, M. Gershenzon: Phys. Rev. 135, A 1399 (1964). [2] Subashiev, V. K., J. Gyulai, G. A. Chalikyan: Report on the Photoconductivity Conference, Odessa, 1965. [3] Gyulai, J., V. K. Subashiev, G. A. Chalikyan: Phys. stat. sol. 17, K49 (1966). [4] Наследов, Д. И., В. В. Негрескул, С. И. Рагауцан, С. В. Слободчиков: Ф. Т. Т. 7, 3671 (1965). [5] Рывкин, С. М.: Фотоэлектрические явления в полупроводниках (Физматгиз, Москва, 1963), р. 40. [6] Subashiev, V. К., S. A. Abagyan: Proc. Intern. Conf. Phys. Semicond. (Dunod, Paris, 1964) p. 225. [7] Bube, R. H.: Photoconductivity of Solids (J. Wiley, New York, 1960) p. 288. О ФОТОПРОВОДИМОСТИ ФОСФИДА Г А Л Л И Я Й. Гьюлаи, В, К. Субашиев, Г. А. Чаликян Была исследована аномальная фотопроводимость очень высокоомных кристаллов фос-фида галлия. Из экспериментальных результатов получается, что аномальная фотопрово-димость связана разными подвижностями в разных минимумах зоны проводимости. П о расчетам было показано, что наличие эффекта самое вероятно в кристаллах с очень малек -кими величинами времени жизни носителей. 

On the photoconductivity of GaP crystals

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On the photoconductivity of GaP crystals

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