Semiconductor interfaces studied by scanning tunneling microscopy and potentiometry

Surface

324

Science 181 (19X7) 324.-332 worth-HolIand. Amsterdam

SEMICONDUCTOR INTERFACES STUDIED BY SCANNING TUNNELING MICROSCOPY AND POTENTIOMETRY P. MURALT

Received

*

14 July 1986; accepted

for publication

30 July 19X6

The potential distribution across the cleaved end-faces of a forward-biased GaAs pn Junction and a &As double heterojunction laser diode were simultaneously mapped with its surface topography. In both cases space charge regions next to the interfaces are visible with nanometer resolution. The potentiometric method shows to be very useful for the localization of heterostructure interfaces.

1. Introduction In this article a new way of studying and characterizing semiconductor junctions is described. Scanning tunneling microscopy (STM) 11.21 combined with pot~ntiomet~ (STP) [3,4] allows semiconductor interfaces to be mapped with nanometer resolution. A GaAs pn junction and a double heterostructure (DH) GaAs laser diode, cleaved under UHV conditions, were chosen to learn about the possibilities of this technique. In a previous letter [3] the method was described and successfully applied for the detection and the imaging of the voltage drop across the high resistive barrier of a planar metal-insulator-metal structure. In the same letter it was pointed out that in case of semiconducting samples the electric potential as measured by STP contains actually dual information: (I) the local electric potential Us(x) and (2) a local material dependent offset potential U,(X). which originates from the asymnletry of the tunnel current-to-voltage (Z,/u,) characteristic. The experimental setup in case of the laser diode is sketched in fig. la. A DC voltage AU = U, - U, is applied across the sample in addition to the voltage U,,, at the tunnel tip (fig. lb). The potential of the sample Us(x) hence varies with position. In crder to distinguish between potential and topographic variations, Ur,, = U,,sinwz is chosen to be an AC voltage. Accordingly, the control electronics has two feedback circuits. One keeps the * Work performed

at IBM Research

Laboratory

Zurich,

(78-8803

0039-4028/87/$03.50 0 Elsevier Science Publishers (North-Hoi~and Physics Publishing Division)

Rtischlikon,

B.V.

Switzerland

AC component of the tunnel current 1, constant and provides the topographic signal. The second one adds an offset Cr,,, to Vi and U, such that the DC component of I, vanishes, i.e. (1,) = 0. The interesting quantity is (Us U,)(x>. The tunnel voltage I&(x,) at the site x0 of the tip,

creates a tunnel current I, - f,( U,), where f,,(V,) is the local tunnel 1JU, characteristics. In the ideal case the DC part of U&x,) is regulated to zero, i.e. UREG(x)+(Us-- U,)(x) =O. When f(U,) is asymmetric (fig. lc) the regulation condition (1,) = 0 requires an offset potential U, of the DC tunnel As a consequence UREo, the source of voltage, i.e. ~~(x~~ = U,,, + Us.

Cl

Fig. 1. (a) Experimental setup with the laser diode structure. (b) Schematic drawing of the surface potential Us for two different positions x0 and xi, of the tip. The actual tunnel voltage is UTIP-t &(x0). (c) Tunnel current-to-voltage characteristic typical for p-type material (p ( ZO”/cm3). A~Ioffset voltage tk is needed to get ( f, > = 0.

326

P. Murult

the potential lJ REG

=

images,

/ STM

amounts

uttd STP

of

to

-(us- W(x) - GL4

(2)

The measurements yielded a positive U, for p-type and a negative UA for n-type material. Hence, the observed change of U,,,; at a pn junction is larger than AU when the junction is operated in forward direction. Us and U, can be distinguished by observing It/U, on an oscilloscope during scanning. An influence of the shape of f(U,) also exists for the topographic scans which are covered by the condition that the rectified AC part (I,,c,,) of the tunnel current (obtained by means of a lock-in technique) is constant and equal to a preset value. The topographic error created in this way, however, is quite small because of the strong exponential dependence of I, on the tip-to-sample separation.

2. Experimental The STM used in the present investigation is of the “pocket-size” type, mounted inside a high-resolution UHV SEM [5]. The sole modification needed was the splitting of the “louse” (step-motor carrying the sample to the tip) and sample holder into two electrically isolated parts. A preparation chamber attached to the SEM allows cleaving of the sample and transferring it to the SEM under UHV conditions (pressure below 5 X lo-” mbar). The pn diode used in these experimentes was grown by molecular beam epitaxy (MBE) on a n-type GaAs wafer. The doping concentrations were NA = N, = 2 X 10’x/cm’. The double-heterostructure (DH) laser diodes were also grown by MBE on an n-type GaAs wafer. The 1.5 pm thick N- and P-Al,,Ga,,,As cladding layers were doped to 5 x 10’7/cm3. The 0.10 pm active layer was nominally undoped GaAs. The junction voltage was provided via TiPt and AuGeNi contacts for the laser diode and In and AuGeNi contacts for the pn diode. The DH structure was clearly visible in the SEM, which was used to guide the electrochemically etched W(111) tip to the position required. The tunnel voltage IF_?,,was usually 2.5 V at about 400 Hz. I,,cIf was usually chosen as 3 nA.

3. pn-diode In fig. sponding junction interface from the

2 a topographic scan image and the simultaneously recorded correpotentiometric scan image of the forward-biased (AU = 0.6 V) pn are shown. The scan direction is 45” inclined with respect to the plane of the junction. The position of the pn junction is not evident topographic image. The voltage drop occurs over a distance of about

of semiconductor interfaces

321

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I Fig. 2. Potentiometric

>

zoon

I

X

and corresponding topographic (top) scan images of a forward-biased pn junction (AU = 0.6 V, p: right hand side).

GaAs

150 A, corresponding to a maximum electric field of roughly 4 X lo5 V/cm. is about 0.4 V larger than the applied bias voltage AU. The drop of Ua,, Measurements of U, yielded values of about -0.1 V for n-type +0.3 V for p-type GaAs. U, increases with decreasing doping concentrations and 1 U, 1 is generally larger for p-type than for n-type material. Most of the potentiometric scan images show the contours of a small “knoll” on the n-side and a weak depression on the p-side. These features are most probably due to the space charge regions, which extend roughly 200 A into p- and n-type GaAs. In fig. 2 the “knoll” is well visible, whereas the depression on the p-side does not show up everywhere or is disturbed by other features. The latter may be due to local variations of band bending at the surface caused by dopants, defects or topographic variations. When the forward bias is reduced to zero the “knoll” on the n-side grows over the whole junction area [6]. The depression on the p-side does not grow or even disappears. It looks like as if the tip potential had to follow the potential of the conduction band minimum in the space charge region of the n-side. STP is obviously much more sensitive to changes of the electron concentration in the conduction band than to changes of the hole concentration in the valence band, owing to the fact that the barrier height between tip and conduction band is smaller than the one between tip and valence band.

P. Murult / STM and STP of semiconductor

328

mtrrfuca

4. Laser diode scan image (fig. 3) shows an atomically flat surface with a The topographic discontinuity in the0 N-AlGaAs region near the interface. The step height varies from 16 to 8 A. (The basic periodicity along the [llO] direction is 4 A.) The tiny vertical steps are either due to variations in the local 1,/U, characteristic or to single steps of 2 A. Again the position of the junction is not evident from the topographic image. However, the simultaneously recorded potential data show a waterfall-like drop from a value close to U, on the n-side to a value somewhat below U, on the p-side. As in case of the pn diode the space charge region of the N-AlGaAs/GaAs interface manifests itself as a small “knoll”. The potential drops mainly near the two heterostructure interfaces, having in between an almost flat plateau. The width between the two drops allows the conclusion that the 100 nm wide active layer is situated between the drops. On P-AK&As the offset potential U, is much larger than the one observed on p-GaAs. This is probably due to the larger gap of Eg = 1.9 V of Al,,Ga,,As. On increasing AU to 1.6 V the “knoll” gets narrower, consistent with a shrinking width of the space charge region. Some new structure appears in the active zone, which must be assigned to potential changes due to electron injection and electron-hole recombination. In addition the drop of U,,,; at

“REG

2-

AU

Fig. 3. Topographic

and potentiometric

image of laser diode

the interface continues the P-AK&As f7], pr&&ly resulting from a non-perfect electron confinement at this interface in the presence of the tip Potential. The situation where discontinuity and heterostructure interface coincide was also encountered (fig. 4). Fig. 4 is a very nice demonstration of how STP can be used to localize electrical interfaces, and hence also heterostructure interfaces. The simple potential image of fig, 4 was obtained by choosing a too long inte~atio~ time of the potential reguiation feedback. As a consequence the DC part of the tunnel voltage is not adjusted to zero (or U,) and prevents the correct m~surement of the topography, which obviously happens. It is interesting to note that the edges of the potential images of figs* 3 and 4 coincide with the bottom of the steps in the topographic images, The space charge regions on the N-sides are not visible on top of the steps. This seems to indicate that STF images the electrical characteristics of the few topmost layers only. When raising the voltage AU across the diode above the GaAs band-gap

Fig. 4. L&m &ode. Feedback of potenti& fegulation with a tong integration time.

value, an increasing number of electrons and holes are injected into the recombination zone. The recombination region moves towards the p-cladding layer [7]. The displacement is 450 A for an increase in junction voltage from 1.4 to 1.8 V. Sample heating and consequent thermal drift prevented an increase of the junction voltage above 1.8 V.

5. Fluctuations of band bending at surface The shape of the I,/ r/, characteristics at given tunnel distance for GaAs( 110) is not yet fully understood. In contrast to other techniques for surface and interface investigations, STM produces locally high density currents, which becomes problematic in case of systems with low densities of free carriers. The applied tunnel voltage partially drops within the semiconductor due to the formation of a so-called spreading resistance region [8]. The strong dependence of the Z/U, characteristic on the carrier concentration could be confirmed observing the change of U* upon switching on the e-beam (30 keV) of the SEM. On P-type (5 x 1017/cm3) Al,,Ga,,,,As, e.g., I!J~ drops from 2.0 to 0.7 V, i.e. the asymmetry of the Z/U, characteristic is partially removed when more free carriers are available. (A similar effect is, of course, observed if GaAs is irradiated with light. In fact, this method allows tunnel experiments to be performed even on samples with low carrier concentrations.) However, spreading resistance alone cannot explain the high voltages needed in case of high doping concentrations. It is observed, for instance, that the tip-to-sample separation (at a given Z,) sharply drops below a certain threshold voltage (DC), which amounts to about 1.2 V in case of p-type material with NA = 10”/cm3. Consistent with that it is reported in ref. [9] that the surface frequently was touched when scanning over a GaAs(ll0) surface with U, < 1.5 V. Band bending by a pinned Fermi level should not occur as the GaAs( 110) surface has no intrinsic surface states within the gap of the bulk material [lo]. However, small amounts of defects and adsorbents may induce a small band bending. The spreading resistance now may become important if the depth of the depletion layer exceeds the mean free path (- 40 A) of the carriers. In addition, the high dielectric constant of GaAs enlarges the external fields required. The band bending is, of course, not homogeneous on the surface. Potentiometric scan images on a cleaved n-type GaAs wafer ( N,, = 2 x 10’“/cm3) show variations of U,,, with amplitudes of about 0.1 V (fig. 5). They occur on atomically flat surfaces. It is well possible that these variations originate from the randomly distributed impurities. Such fluctuations of the same order of magnitude were derived from tunneling spectroscopy [ll] at GaAs(lOO) Schottky barriers.

P. Mural1 / STM and STP of semiconductor interfaces

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Fig. 5. GaAs(ll0)

6.

surface of a nominally

,

331

,

,

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n-type wafer.

Conclusions

The different experimental findings indicate that local information on electronic transport in semiconductors can be obtained with nanometer resolution. STP allows destruction-free measurements and correlates potential information with surface structure information. The method may help to gain a better understanding of the problems involved with Schottky barriers, surface charges, Fermi level pinning and related phenomena occurring at semiconductor interfaces. The potentiometric accuracy of STP is better for metals (U’ -=K fit1 < AU) than for semiconductors (U, < fit, > AU), because the amplitude of the AC tunnel voltage (fit,) must be larger than the applied voltage AU across the samples in the case of semiconductors.

Acknowledgements I would like to thank H. Meier for providing the samples, A. Baratoff, H.W. Fink, V. Graf, Ch. Harder, D.W. Pohl and H.W.M. Salemink for useful discussions, and H. Rohrer for continuous support.

P. Murali / STM and STP

332

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References [l] [2] [3] [4] [5] [6] [7] [S] [9] [lo] [ll]

G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Appl. Phys. Letters 40 (1982) 17X. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Letters 49 (1982) 57. P. Muralt and D.W. PohI, Appt. Phys. Letters 48 (1986) 514. P. MuraIt, D.W. Pohl and W. Denk, IBM J. Res. Develop. 30 (1986) 443. Ch. Gerber, G. Binnig, H. Fuchs, 0. Marti and H. Rohrer, Rev. Sci. Instr. 57 (1986) 221. P. Muralt, to be published. P. Muralt, H. Meier, D.W. Pohl and H.W.M. Salemink, to be published. F. Flares and N. Garcia, Phys. Rev. B30 (1984) 2289. R.M. Feenstra and A.P. Fein, Phys. Rev. B32 (1985) 1394. W. Gudat and D.E. Eastman, J. Vacuum Sci. Technol. 13 (1976) 831; A. Huijser, J. van Laas and T.L. van Rooy, Surface Sci. 62 (1977) 472. J.W. Conley and G.D. Mahan, Phys. Rev. 161 (1967) 681.