Negative photoconductivity in modulation-doped quantum wells

Superlattices and Microstructures, VoL 5, No. 1, 1989

NEGATIVE

PHOTOCONDUCTIVITY

15

IN M O D U L A T I O N - D O P E D

QUANTUM

WELLS

R.A. Hopfel, S. Juen

Institute of Ezperimentai Physics University of Innsbrnck, A-60~O Innsbrnck, AUSTRIA J. Shah

A T& T Bell Laboratories, ttolmdel, NJ 07785, USA A.C. Gom~rd

Department of Physics University o/California, Santa Barbara, CA 95106, USA (Received 12 August 1988)

Three novel photoconductivity effectsin modulation-doped quantum wells are described, which all lead to negative photoconductivity at low temperatures: (1) Extreme increase of the in-plane resistance in p--doped GaAs/AIGaAs quantum wells (up to a factor of > 60) with illumination is observed, due to hole trapping in the potential minima of AIGaAs and subsequent recombination of minority electrons. (2) Negative photoconductit~ty due to **carrier drao" leading to "negative absolute mobility** of minority electrons is ezperimentally studied. (3) In n--
INTRODUCTION: Modulation-doped quantum wells of GaAs AIGaAs reveal high majority carrier mobilities (exceeding 5 x I05 cm2/Vs for electrons and I x 106 cm2/Va for holes) t. At the same time electrons and holes are confined within the thin GaAs layer, which is relevant especially for optical applications. Both properties together have lead to the recently observed effect of **negative almolute mohility'S: For electric fields applied in plane of the wells, the high--mobility m~ority carriers ndrng" the injected minority carriers in the "wrong" field--
time the intrinsic negative photoconducti~ity due to electron-hole drag causing negative absolute mobility.

0749-6036/89/010015 + 04 $02.00/0

I. p--DOPED QUANTUM WELLS: In the experiments, we used F-modulation-doped quantum wells cqumating of 20 periods of undoped Gabs (dl = 112 A), Be-doped Alo.4sG~o.6~ (d2 = 49 A), and, undoped Al0.uGa0.6p~s *'spacer** layers (d8 = 294A) between the doped regions and the GaAs quantum wells (see inset of Fig. l). The holes from ionized acceptors in AlOaAs form a high-mohility hole plasma in the GaAs, with a concentration of p, = 1.5 x 10It cm "1 (per layer) and a mobility pp = 53 800 cm2/Vs (at 4.2 K). The values at 77 K are p= = 1.83 x 10II cm"s, #p = 3 750 cmS/Vs. The photoconductivity experiments are performed by shining low intensity monochromatic light (from a tungsten halogen lamp with chopper through a 1/4 m monochromator) on a 4--point sample geometry. Constant current is applied to the outer contacts, the voltage at the two tuner contacts is measured. The light intensity was of the order of 3 x 10"s W/cm 2. Fig. 1 shows the results at low temperature (T = 20 K) and at room temperature. The change of sheet condttctivity (in A/V) of the whole system of 20 quantum wells is plotted as a function of the wavelength of the incident light. The spectra are taken with low applied electric fields, m the linear transport regime. The following features are important: At low temperature (T = 20 K), the photoconductivity is negative in the whole spectral range. The photoconductivity signal starts slightly above the bandgap of GaAs, increases with decreasing wavelength, and shows a sharp rise around 580 nm to © 1989 Academic Press Limited

16

Superlattices and Microstructures, VoL 5, No. 1, 1989

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The other spectral features axe discussed extensively in Ref. 4, we emphasize here, that with hi.gher light intensities an increase of the sample resmtance by a factor of up to 60 has been observed. This corresponds (assuming a linear dependence of mobility on carrier concentration s) to a decrease of the hole concentration to about 20 % of the equilibrium value. This should have applications not only for optical experiments with p-modulation--doped quantum well systems: The high negative photoconductivity at low intensities (5 A / V per W/cm~) means high responsivities for detector applications.

II. NEGATIVE MOBILITY (CARRIER DRAG): lo ~ 400

a 500

t

700 rim, &X= t 5 nm (FWHM}

600 700 WAVFLENGIH (rim)

800

900

FIG. 1: Spectral photoconductivity Aa (=absolute change of sheet conductivity of the whole system of 20 p-doped quantum wells) as a function of wavelength.

Inset: Sample structure (band edges) and the physical mechanism leading to negative photoconductivity.

a huge maximum at 560 nm. The sharp rise is at a photon energy slightly above both the direct and indirect bandgaps of A10.48Ga0.s~ks. In contrast, at room temperature the photoconductivity is positive, much smaller(note the change of scale by -10 4), and shows no specific structure above the GaAs bandgap. We interpret the data as follows: At low temperatures, optical injection of carriers above the bandgap of AIOaAs leads to a complex process as indicated in the inset of Fig. 1. Electron-hole pairs in A1OaAs (1) are separated in the strong electric field of the "spacer" layer, between ionized acceptors and the hole plasma in the quantum wells. Holes are driven into the potential minima in AlGa.As (2), whereas the electrons are driven into the quantum wells (3). In the quantum wells the electrons recombine (4) rapidly with holes due to the high hole concentration. The recombination time in these samples is measured as 1.0 as (Ref.3). The wholeprocess restdts in a decrease of the hole concentration (and therefore conductivity), until the trapped holes are thermally excited again into the quantum wells (5), and the equilibrium situation is given. This process can cause large decrease of the conductivity, if the trapping time of the holes in A1GaAs is long compared to the recombination time of the electrons in GaAs. From the data, using a simple rate equation for the detrapping, a trapping time of about 0.5 ms is obtained, in agreement with direct experimental observation. The trapping time, of course, depends on the several parameters involved: It decreases with increasing temperature (thermal emission), and it decreases with increasing light intensity. The latter effect is expected, since at high hole concentrations in AIGaAs, the band bending is electrostatically reduced at high hole trapping densities.

As described above, the low temperature photoconductivity in p--doped quantum wells is dominated by the trapping of holes in AlGa.As, which leads to a decrease of the hole concentration in the quantum wells (through the recombination of minority electrons). Also well below the bandgap of AIGaAs, at low light intensities, negative photoconductivity has been experimentally observed4, which, however, is by orders of magnitude larger than expected from the electron-hole scattering times. There is no satisfying explanation for the negative photoconductivity in this range (Fig. 1, 680 n m < A < 820 nm) 4. Since the drag effect (negative mobility) has been experimentally observed2 at much higher minority injection levels (> 101° cm-2), we performed photoconductivity experiments in this spectral range with comparable excitation levels, much higher than in Ref. 4. In these experiments we used the identical sarnple as in Kefs. 2 and 4 (see chapter I), with a 3 mW A1GaAs diode laser as the light source(A = 750 nm, periodically modulated by the current). The light intensity on the sample was in the order o f l 0 W/cm~. Additional white light (tungsten lamp, not modulated) was focussed on the sample in order to saturate possible trapping effects within the GaAs quantum wells. By correlation measurement (lock-in amplifier), only the photoconductivity due to the AIGaAs laser is measured. In Fig. 2 the results are shown, for two different values o f the additional white light: The photoconductivity at low temperatures is negative up to a temperature of 80 K, at higher temperatures the signal is positive. As shown in the inset, a transition from negative to positive photoconductivity is observed also as a function of electric field (at the lowest sample temperature), which leads to heating of the majority hole plasma. The temperature dependence of the photoconductivity agrees roughly with the theoretically predicted behaviour (dashed curve). In Ref. 3 it has been shown that the photoconductivity becomes negative exactly when the mobility of minority carriers becomes negative. Quantitativety, for a semiconductor with equilibrium electron and hole concentrations no and P0, the change of conductivity by injection of n - no (= p - P0) electron-hole pairs is given by A a = p.e./~h + n-e'/Je-- p0"e'#h-t -- n0"e'#e-I

(1)

Superlattices and Microstructures, Vol. 5, No. 1, 1989 The minority carrier concentration is known from the light intensity and the recombination time of 1.0 ns, which has been experimentally measured by time-resolved luminescence 3. The values for the scattering times are known from the optical transport experiments 3. The dependence of the measured photoconductivity on the electricfield (inset of Fig. 2) is consistent with the transition from negative to positive mobility due to majority carrier heating, as observed in Ref. 2. Therefore we claim that the

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observed negative photoconductivity is due to the negative absolute mobility of the minority electrons. The quantitative agreement of our data with the

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FIG. 2: Photoconductivity of the p--doped quantum well structure (normalized to unit light intensity of 1 W/crn2), as a function of temperature. Light source: AIGaAs laser diode A = 750 nm, intensity 10 W / c m 2. Background white light intensity: lwl = 260 and 340 m W / c m 2, respectively. Dashed line: Theoretically predicted values. Inset: Photoconductivity as a function of electricfield.

The mobilities #h and #e are the hole and electron mobilities in the presence of photoexcited carriers (i.e.,including the influence of electron-hole scattering). #h-I and Pe-I are the mobilities without photoinjection of carriers - determined only by hole-lattice and electron-lattice scattering, respectively. Using the expressions for #h and #e as given in Ref 3 ("hydrodynamic" approximation), we obtain A~ =

17

P'e2 1 + p ' m h ' < r ~ - h > + p.mh' n ' m e ' < ~ ' h - I> n.me. • [_P0"< rh4> + < r ¢ _ h > . ( p _ ~ L p - mh me + n -- n O . m__.hh. ) + n me + .(p_._~+ n - no_ me p _ 2 _ n o . mh n m---; " <~h-,> ) ]

+

(2)

(mh and me ... effective masses, ... momentum relaxation time of electrons relative to the hole plasma by electron-hole scattering, and ... momentum relaxation times of electrons and holes by lattice-scattering). In figure 2, A a ~cording to the equation above, normalized to unit light intensity (1 W/cm 2) and multiplied by 20 (number of quantum well periods), is plotted as a fimction of temperature (dashed line).

theoretically predicted behaviour is not fully satisfactory. The most critical parameter for the comparison is the minority carrier concentration during laser illumination, which is determined by the recombination lifetime. For the comparison in Fi~. 2, we used a value for < tree> of 1.0 ns, as known Irom the time--resolved luminescence3. This value, however, was measured at minority electron concentrations of 3 x 1010 crn-2. The photoconductivity measurement, in contrast, is performed at lower light intensities, causing electron concentrations in the order of 10a cm-2. The lifetime at these low electron concentrations might be longer, due to trapping or due to aasymetry of the quantum wells, leading to a smaller overlap of the electron and hole wave functions, which is screened at higher excitation densities. The data clearly show better agreement at higher background light intensities. Therefore, we shall continue the experiments at higher minority electron concentrations. III. n--DOPED QUANTUM WELLS: In the experiments with n--modulation--doped quantum wells, samples wsre used with 15 periods of undoped Ga~s (dl = 258 A), Si--doped Al0.2sGa0.zrAs (d2 = 284 A), a0d undoped A10.23Ga0.77As "spacer" layers (de = 118 A) between the doped regions and the GaAs quantum wells. The electrons from the ionized donors in A1GaAs form a high-mobility electron plasma in GaAs, with a concentration of n, = 3.0 x 10" cm-2 (per layer) and a mobility of 102 000 cm2/Vs (at 4.2 K) and 69,000 cm2/Vs (at 77 K). The hght intensity was of the order of 3 x 10-4 W/cm 2. The sample temperature was around 50 K. We obtain the following typical results, as shown in figure 3. Without background illumination (curve "dark"), at photon energies above the bandgap of GaAs (A < 830 nm) high positive photoconductivity is present. Towards shorter wavelengths (note the change of scale at A = 725 nm) the photoconductivity stays approximately constant, and shows some weak but reproducable spectral structure. A sharp rise occurs at a wavelength of 690 nm, which coincides with the expected bandbap for Al0.~Ga0.rrAs. At wavelengths below the bandgap of GaAs ~L>s 830 am) negative photoconductivity is observed, negative peak (at A = 833 nm) becomes larger when some background illumination is turned on, An even more pronounced increase of the negative peak is present when the temperature is raised to 70 K (dashed curve)~ Furthermore (not shown), the negative peak increases with electric field.

18

Superlattices and Microstructures, Vol. 5, No. 1, 1989

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photon with the right energy therefore can excite an electron from the valence band to the deep donor state, leaving behind a hole which recombines with an electron from the conduction band (directly or indirectly via traps). If the lifetime of the electron in the deep donor state is longer than that of the hole, the net number of electrons in the conduction band is decreased. This model is consistent with our observations of the dependence on illumination, electric field and temperature. All three influences increase the number of ionized donors and thus the

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ACKNOWLEDGMENTS:

600

W e thank Prof. P.A. Wolff, Cambridge, for valuable discussions. T w o of us (R.AH., S.J.) acknowledse support by the "Fonds zur F5rderung der wimsenschaftlichen Forschung (FWF)", Austria, project P6184.

FIG. 3: Spectral photoconductivity (per layer) of

n-modulation-doped

quantum

wells, with

and

without background illumination. The daRhed curve is

measured at higher temperature.

To explain the observed negative effect, we propose the following model as discussed in detail in Ref. 6: Deep donor states in AIGa/ks near the interface or in GaAs are occupied by electrons since the Fermi energy is about 20 m e V above the conduction bandedge. The donors can be ionized by light, by electric field or by increasing the temperature. A

REFERENCES:

1 J.H. English, A.C. Gc~ard, H.L. St6rmer, and K.W. Baldwin, Appl. Phys. Lett. 50, 1826 (1987). 2 R.A. HSpfel, J. Shah, P.A. Wolff, and A.C. Goa~rd, Phys. Rev. Lett. 56, 2736 (1986). s R A HSpfel, J. Shah, P A . Wolff, and A.C. Go~ard Phys. Rev. B37, 6941 (1988). 4 R.A. HSpfel, Appl. Phys. Lett. 55, 801 (1988). s D.C. Tsui, A.C. Go~ard, G. KmninJky, and W. Wiegmann, Surface Sci. 113, 464 (198'2). e R.A. HSpfel, Appl. Phys. Lett. 5I, 106 (1987).