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Submitted on 1 Jan 1987
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VERY LARGE PHOTOVOLTAIC EFFECTS IN MODULATION-DOPED HETEROSTRUCTURES
M.-H. Meynadier, N. Tabatabaie, R. Nahory, J. Harbison
To cite this version:
M.-H. Meynadier, N. Tabatabaie, R. Nahory, J. Harbison. VERY LARGE PHOTOVOLTAIC EF-
FECTS IN MODULATION-DOPED HETEROSTRUCTURES. Journal de Physique Colloques, 1987,
48 (C5), pp.C5-187-C5-190. �10.1051/jphyscol:1987537�. �jpa-00226741�
VERY LARGE PHOTOVOLTAIC EFFECTS IN MODULATION-DOPED HETEROSTRUCTURES
M.-H. MEYNADIER, N. TABATABAIE, R.E. NAHORY and J.P. HARBISON Bell Communications Research, Red Bank, NJ 07701-7020, U.S.A.
Nous avons mis en evidence l'existence d'un effet photovoltaique dans les heterojonctions a dopage module de faible densite bidimensionelle. Des photovoltages lateraux de l'ordre de la centaine de millivolts apparaissent entre deux contacts ohmiques distants d'environ 1 cm lorsqu'un faisceau laser de quelques microwatts est focalise sur la surface de l'echantillon. L'effet est attribue a la separation des porteurs dans la region de l'heterojonction. L'intensite du photovoltage depend fortement de la position du spot laser par rapport aux contacts, et sature pour les larges puissances d'excitation, ce qui rend cet effet directement utilisable pour un dispositif optoelectronique.
We report the observation of large in-plane photovoltages in low electron density modulation doped heterostructures. Local optical excitations of the order of microwatts are found to produce lateral photovoltages as high as several hundred millivolts between contacts centimeters away from the exciting spot. The effect is attributed to the vertical photovoltage locally induced by the separation of the photocreated electrons and holes. Its strong position dependence as well as its saturability are shown to be attractive for technological applications.
Modulation doping of semiconductor heterostructures is now a well controlled technique for creating two-dimensional (2D) sheets of electrons or holes [1,2]. While improvements in growth conditions keep increasing the mobility of such 2D gases, a considerable amount of both physics and device work has been generated by these new structures. Until recently, most of these studies investigated o r made use of the unique transport properties of a degenerate two dimensional electron gas (2DEG) in the absence of scattering impurities. Fewer optical experiments have been canied out, although optical generation of electron-hole pairs has been found to affect dramaticaily both the mobilities and electron densities of these structures [3-51.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987537
C5-188 JOURNAL DE PHYSIQUE
In the present work we demonstrate that local optical illumination of a 2DEG leads to new effects, especially if the electron sheet density is very low. The samples used are GaAs/GaAlAs modulation doped heterostructures grown by molecular beam epitaxy. They c o n s i s t l ~ f a emi-insulating substrate on which are sequentially grown a 4 pm undoped ( p ~ 10 cm-
's
) GaAs buffer 400 A Ga 7A1 3As undoped spacer, a 400 A Ga 7A1 3As:Si layer (n in the lOQk'' range) and 170. A GaAs cap layer. Two o r more ohmic 'contacts separated by about one centimeter are made on the sample edges. A photovoltage is found to build up between these contacts when a HeNe laser beam is focussed to a spot about 50 pm in size anywhere on the sample surface. The magnitude of this photovoltage at room temperature is of the order of a few tens to a few hundreds of millivolts for microwatts to milliwatts of incident power, depending on the sample structure.The photovoltage saturates for intensities higher than a few milliwatts. Furthermore, its intensity and sign are very sensitive to the position of the exciting spot with respect to the two contacts, as can be seen in Figure 1. It is found to increase quasi-linearly from -V to
+
V when the spot is moved from one contact to the other, with a zero crossing at the midpoint.The spectral response of this effect shows that it turns on for incident photon energies greater than the GaAs bandgap. Hence it is associated with photocreation of caniers in the GaAs buffer layer.
Y
a 2 0
0 )
a 0
-
0; - 2 0 +
0 C
P - 4 0 4
s p o t p o s i t i o n
( m m )
Figure I : Measured photovoltage vs position of the optical excitation spot (50 pm diameter, x I mW HeNe) along a line between two ohmic contacts 8 mm apart. The zero position is the center point of this line.
One can understand the essential nature of this phenomenon the following way: due to the surface proximity and the thickness of the spacer in our samples, most of the electrons introduced by doping the GaAlAs layer are actually transferred to surface states, and do not participate in the conduction. This results in a non-degenerate 2DEG at the GaAs/GaAlAs interface at room temperature. Hall and resistivity measurements performed in the dark at 300K show that the Fermi energy level lies a few tens to a few hundreds of meVs below the first conduction subband. Away from the 2DEG, the Fermi level is fixed by the low residual acceptor concentration , hence also well inside the band gap. In the presence of light, GaAs photocamers are created and separated by the existing electric field, resulting in a raising of the Fermi level in the 2DEG region and a lowering in the buffer region, similar to a p-n
of this effect is expected to take place due to (i) saturation of the 2DEG number of camers, which occurs for large enough sheet densities by transfer of electrons back to the bamer layer, and (ii) reduction of the thickness of the electric field region
in the GaAs layer for incre ing 2D G densities. This is found experimental1 to o cur fo
Y
I%a
excitation levels around 10 W/cmf: which corresponds to an estimated 10''-I0 cm- electron density. This.mode1 is also corroborated by the very low photovoltages found at lowcr temperatures (in the mV range for T < 30K), at which the Fermi level is expected to be pinned in the conduction subband and is thus less sensitive to a light-induced increase of camers.
Remarkably, this vertical photovoltage can be sensed laterally centimeters away from the two electrical contacts. This can be somewhat simplistically understood by the equivalent circuit drawn in Figure 2, in which a is a dimensionless parameter measuring the relative distance of the spot to the contacts, and R and R' the in-plane resistances of the 2DEG and GaAs buffer layer, respectively. Also p is a vertical resistance which includes both the 2DEG leakage resistance due to thermionic emission and the n-type ohmic contact access resistances to the p-type layer. The excited region is pictured here by a battery of voltage v, to which for more detailed analysis a series resistor might need to be added.
However, for present purposes this model suffices.
GaAs b u f f e r
-
S.I. G a A s substrafe-
Figure 2: Sample structure and equivalent circuit. I and 2 are n-type ohmic contacts between which the photovoltage is read.
The resulting photovoltage measured between points 1 and 2 reads then (2a-1)pR
V I 2 - -
... *
v[p
+
(I-a)(R+
R')] [p+
a(R+
R')]Hence, in agreement with our experimental results, the photovoltage appears negative at the contact the closest to the optical excitation, increases to the opposite value towards the other contact, and cancels for a = 0.5.
C5-190 JOURNAL DE PHYSIQUE
Although self consistent charge transfer calculations could a priori be done to describe more quantitatively this effect [6], they would be only marginally reliable in view of the importance of the surface states (of unknown density) in the depletion process, and of the difficulty to evaluate the channel density of a strongly cornpensated 2DEG in the dark. Our simple model provides a satisfactory description of the magnitude and position dependence of the observed photovoltage.
It is worth pointing out that this phenomenon provides a rather efficient type of position sensing mechanism. A square device of the order of a centimeter in size with electrical contacts at the edges would be able to distinguish the location of a 100 pm spot impinging anywhere on the surface. Furthermore, the combination of the position sensitivity with the saturation behavior has been used to implement simple logical functions (and, or, exclusive or) by shining 2 spots at selected positions. Obviously the system is able to distinguish much' more than two spots, especially if one made use of more than one pair of contacts. This could lead to interesting applications for optical computing purposes. The speed is of the order of microseconds for the entire photovoltage to build up, which is reasonable in view of the fact that numerous bits of information can be treated simultaneously. A specific coding scheme is however beyond the scope of the present report.
In conclusion, we have demonstrated the existence of a large in-plane photovoltaic effect in GaAsJGaAlAs modulation doped heterostructures of very low 2DEG densities. The phenomenon is understood in terms of a vertical photovoltage induced by the separation of carriers, and is sensed laterally through the in-plane resistances of the 2DEG and the GaAs buffer. Useful applications of the position sensitivity and saturability of this effect are currently under investigation.
Acknowledgements
We are grateful to M.D. Sturge and J.M. Worlock for useful discussions.
References
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[3] H.L.Stormer, A.C. Gossard, W. Wiegmann and K. Baldwin, Appl. Phys. Lett. 39 , 912 (1981).
[4] A.S. Chaves, A.F.S. Penna, J.M. Worlock, G. Weimann and W. Schlapp, Surf. Sci. 170, 618 (1986).
[ S ] C. Delalande, J. Orgonasi, M.H. Mcynadier, J.A. Brum, G. Bastard, G. Weimann and W.
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