Probing the Wurtzite Conduction Band Structure Using State Filling in

LETTER pubs.acs.org/NanoLett

Probing the Wurtzite Conduction Band Structure Using State Filling in Highly Doped InP Nanowires Jesper Wallentin,*,† Kilian Mergenthaler,† Martin Ek,‡ L. Reine Wallenberg,‡ Lars Samuelson,† Knut Deppert,† Mats-Erik Pistol,† and Magnus T. Borgstr€om† † ‡

Solid State Physics, Lund University, Box 118, S-221 00, Lund, Sweden Polymer & Materials Chemistry/nCHREM, Lund University, Box 124, S-221 00, Lund, Sweden

bS Supporting Information ABSTRACT: We have grown InP nanowires doped with hydrogen sulfide, which exhibit sulfur concentrations of up to 1.4%. The highest doped nanowires show a pure wurtzite crystal structure, in contrast to bulk InP which has the zinc blende structure. The nanowires display photoluminescence which is strongly blue shifted compared with the band gap, well into the visible range. We find evidence of a second conduction band minimum at the gamma point about 0.23 eV above the band edge, in excellent agreement with recent theoretical predictions. Electrical measurements show high conductivity and breakdown currents of 107 A/cm2. KEYWORDS: Nanowire, MOVPE, photoluminescence, doping, wurtzite

O

ver the past two decades, advances in the growth of semiconductor nanowires (NWs) have allowed demonstration of various devices of both technical and scientific interest.1
3 Although the common III
V materials have the zinc blende (ZB) crystal structure in bulk, NWs commonly show a mix of ZB and wurtzite (WZ) crystal structure.4 The lower symmetry of the hexagonal WZ leads to different optical selection rules than the cubic ZB, as demonstrated both theoretically and experimentally.5 The WZ conduction band in InP has Γ7 symmetry while the lowest-energy valence band has Γ9 symmetry, allowing only optical transitions polarized orthogonal to the c axis, i.e., the NW growth axis. De and Pryor recently predicted that WZ InP should have an unusual conduction band (CB) structure with a second minimum (Γ8) at the gamma point only 0.24 eV above Γ7.6 This band structure could be important for both optical and electrical characteristics, and, e.g., in bulk InP interband scattering leads to the so-called Gunn effect with negative differential mobility at high fields.7 In this Letter, we show that in situ doping by hydrogen sulfide (H2S) creates pure WZ InP NWs with high S doping. Using microphotoluminescence, we find a strong blue shift as the carriers fill the CB. We take advantage of the strong state-filling to probe the CB structure and experimentally verify the predicted Γ8. InP NWs were grown with molar fractions of H2S (χH2S) between χH2S = 0 and χH2S = 1.11  10
5. The NWs were investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM) including energy dispersive X-ray spectroscopy (EDX), single NW photoluminescence (PL), as well as with electrical measurements using NW field effect transistor (NW-FET) devices. More experimental details can be found in the Supporting Information. r 2011 American Chemical Society

The addition of H2S strongly increased the growth rate, as shown in Figure 1C, resulting in about 3 times longer NWs at the highest χH2S. The radial growth rate decreased from 10% of the axial growth rate to about 1%, but the NW volume increased by around 50%. This indicates that the use of H2S suppressed growth on the substrate and the NW sidewalls and thereby increased the material available for axial NW growth. S is a known surface passivator for InP and it has been shown that InP surfaces treated in situ with H2S are covered with In
S bonds.8 Note that the last doubling of H2S only induced a small increase in growth rate, which suggests fully passivated surfaces. TEM analysis showed that the undoped reference NWs had a mixed crystal structure with approximately equal shares of ZB and WZ. At χH2S = 5.6  10
7 the wurtzite fraction increased to 79%, and in the most highly doped NWs only one stacking fault was found in a total of five investigated NWs. This demonstrates that H2S induces growth in WZ crystal structure in InP NWs, which confirms previous indications9,10 and mirrors the p-type dopant DEZn which induces ZB.11 The smooth sidewalls on the pure WZ NWs may offer fewer preferential nucleation sites for radial growth, enhancing the S passivation effect of increased axial growth. NWs grow in a layer by layer mode, where a nucleus forms and the layer is completed by step flow growth. The crystal structure is determined by the nucleus, and the step flow growth is completed before the next nucleation.12 The solid
vapor surface energy, γSV, is expected to be lower for a WZ nucleus compared Received: February 10, 2011 Revised: April 29, 2011 Published: May 23, 2011 2286

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Figure 1. (A and B) HRTEM of InP NWs grown with χH2S = 0 and χH2S = 11.1  10
6, respectively. Scale bars 10 nm. Insets show SEM of the NWs, scale bars 50 nm. (C) NW length and radial growth vs χH2S. (D) Wurtzite crystal structure, as percent of total, and NW diameter, vs χH2S. (E) S content measured by EDX, and S content normalized by NW length, vs χH2S. Inset shows scan orthogonal to the NW axis.

to a ZB nucleus, due to fewer dangling bonds on the WZ surface, and this is considered the basic driving force for WZ formation.13,14 We measured the diameter of the NWs since it is an indication of the contact angle of the seed particle during growth.15 The gradual decrease in NW diameter from H2S exhibited in Figure 1D shows that the contact angle during growth increased, which implies that the ratio γLS/γLV increased. This demonstrates that the surface energies changed, which is not unexpected since S is a surface passivator of liquid metals known to lower the liquid
vapor surface energy γLV.16 S also strongly improves the wetting of In on ZnS and ZnSe,17 indicating a lowered liquid
solid surface energy γLS. If both γLS and γLV decreased due to the interaction with H2S, the solid
vapor interface would become comparatively more important which could promote WZ formation.13 The increased growth rate indicates an increased supersaturation in the seed particle, which also could promote WZ formation.13

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Since the growth rate is exponentially dependent on the steadystate supersaturation, the large increase in growth rate observed here may be due to a small increase in supersaturation. Furthermore, Glas et al. recently showed that the supersaturation can be self-regulated.18 Thus our measurements do not conclusively reveal if the changes in crystal structure are due to changes in supersaturation or surface energies. These effects are not mutually exclusive, and the quick and complete transition to WZ within a narrow range of χH2S indicates synergetic rather than competing effects. The S concentrations were high enough to be measurable by EDX, as shown in Figure 1E. For the highest χH2S, the EDX measurements showed a S content of 0.9%, with a standard deviation of 0.2%. The measured S content increased sharply around χH2S = 0.56  10
6, coinciding with a sharp increase in WZ fraction and a decrease in radial growth. A possibility is that around this χH2S, the surface became fully passivated with In
S bonds. Line scans across the NWs indicated that the S concentration was higher at the surface, especially at the NW base. The EDX measurements seem to indicate saturation in the S content versus χH2S, but the coinciding increase in growth rate could have diluted the S. We calculated a S concentration normalized with the NW lengths, and as can be seen in Figure 1E this normalized S concentration increased vs χH2S throughout the series. At the highest χH2S the limit of our source was reached. We therefore decreased the NW growth rate by lowering the TMI molar fraction by 40%, and reached a S concentration of 1.4%. The lower TMI flow also increased the radial growth which prevented direct comparisons using PL. We investigated the NWs in the H2S variation series using PL, shown in Figure 2, as well as PL excitation spectroscopy (PLE) of single NWs at liquid helium temperature. The undoped NWs showed luminescence in the range between the ZB and WZ band gaps of 1.42 and 1.49 eV.5,19
21 The spectra of the undoped NWs showed a strong blue shift with increasing excitation intensity, as previously observed in similar NWs.22 With increasing doping, the PL showed increasing broadening at both higher and lower energy. Also, the overall signal strength gradually decreased, presumably due to reduced crystal quality from the high impurity concentrations, while the excitation power-dependent blue shift disappeared. The luminescence extended far below the band gap which we attribute to exchange interaction between carriers and Coulomb interaction of the carriers with the ionized impurities.23 The luminescence also extended far above the band gap, with the highest doped sample having most of the signal in the visible range. This is explained by a strong Burstein
Moss (B-M) shift, as the high density of free carriers fill the available states and shift the Fermi level far above the conduction band edge.23 Comparable B-M shifts have previously been observed in bulk InP24 and InP NWs.25 There was a strong qualitative difference between the NWs grown with χH2S = 1.9  10
6 and χH2S = 3.9  10
6. PL from the former sample showed a slight plateau around the band gap at 1.49 eV, A, and a single peak, B, while the latter one exhibited an additional peak, C. PLE of these two samples (see Supporting Information) showed single absorption edges, at a higher energy for the higher-doped sample. This is consistent with a Fermi level which is shifted increasingly far above the conduction band edge. We investigated the sample with χH2S = 3.9  10
6 more thoroughly, paying special attention to the relative intensity of the peaks B and C. The spectral shape did not show any excitation power dependence. With increasing temperature up to 2287

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the NW axis. The higher-energy peak, C, was the most strongly polarized with a ratio of parallel to orthogonal intensity of about 40%. The line shape from B-M shift reflects the transition probability as well as the density of states in the conduction band,23 and a single conduction band should not give rise to two distinct peaks. WZ InP has three quite closely separated valence bands which have been observed using PLE.20,21 Although it is possible that transitions from the Fermi level to the higher-energy valence bands could be seen in PL at high excitations,19 these should then be observed at all doping levels since the varying electron concentration primarily affects the conduction band. Furthermore, such a transition should be sensitive to the excitation intensity,19 in contrast to our observations of constant relative peak intensity. A second gamma point, Γ8, has been predicted in WZ InP as low as 1.712 eV.6 The same paper predicted a WZ band gap of 1.474 eV, slightly lower than the experimentally determined values.5 Thus, the high-energy peak, C, in our PL spectra is in agreement with the predicted second gamma point, and we interpret the PL at χH2S = 3.9  10
6 as simultaneous emission from two CBs. From group theory it is expected that the transition from Γ8 to the highest VB (Γ9) should have the same polarization as the transition from Γ7 to the highest VB, i.e., orthogonal to the NW axis, which agrees with our measurements. The middle peak, B, is related to the excited carriers far away from the band edge, and it has been shown that these transitions do not comply well with k-selection rules.23 Presumably these transitions also comply less strictly with the polarization selection rules, resulting in slightly less polarized light. Note that the dielelectric confinement enhances the light polarized parallel to the axis5 and that transitions from Γ7 to the lower VBs (Γ7) should not be polarized. For large NWs the dielectric confinement can give rise to a complicated geometrical dependence of the polarization,26 but our NW diameters are only 10
15% of the wavelength of the light. The B-M shift can be used to measure the carrier concentration. In a simple parabolic-band model, the shift is given by23 !  2=3 p2 16:9 n 2 2=3 ΔE ¼ ðmeVÞ ð3π nÞ  meff 1019 cm
3 2meff ð1Þ

Figure 2. (A) PL of single NWs grown with varying χH2S. (B) Polarization-dependence of a triple-Gaussian fit of PL from a NW grown with χH2S = 3.9  10
6. (C) Sketch of the WZ InP band structure as predicted in ref 6. (D) Evaluation of the PL spectra as function of χH2S. The error bars show standard deviations for the 5
10 NWs which were evaluated at each χH2S. The one-band model is E = 1.49 þ 1100χH2S2/3 (eV), while the two-band model is E = 1.70 þ 280χH2S2/3 (eV).

80 K the overall signal weakened but the shape of the spectra remained the same. We performed polarization measurements and fitted the spectra with a triple-Gaussian model. The analysis (Figure 2B) showed that the PL was polarized orthogonal to

Here, meff is the electron effective mass, which for wurtzite InP (Γ7) has been calculated by De and Pryor6 to be about 20% higher than ZB, and n is the carrier concentration. We evaluated the peaks as well as the half-maximum of the high-energy edge, D, as functions of χH2S, shown in Figure 2C. Above χH2S > 1.9  10
6 the peak B remained constant, while the peak C and the edge D increased. At the same time the peaks became a bit less well separated and the signal got weaker. Following the discussion above, we interpret the blue shift of the right peak C as a B-M shift in the second CB. The predicted higher effective mass at Γ86 would reduce the B-M shift in this CB. We have plotted two theoretical models in Figure 2D, both using eq 1 and assuming that n is proportional to χH2S. The models assume band edges at 1.49 and 1.70 eV, respectively, and have different proportionality constants. The first model shows a decent fit for moderate doping levels, up to χH2S = 3.9  10
6. At low χH2S the crystals contain short ZB segments which allows for transitions below Γ7. At higher doping levels the second CB can contribute to the DOS, reducing the blue shift. A reasonable 2288

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Nano Letters fit was achieved in this region with the second model, using a proportionality constant which was about 4 times smaller, although the data points are too few to make a proper analysis. Nevertheless, considering the simple model this agrees remarkably well with theoretical predictions of the relative effective masses of Γ7 and Γ8.6 The parabolic-band model ignores the band gap shrinkage and the nonparabolicity of the bands, effects which will both reduce the B-M shift at very high doping levels but which are unknown for WZ InP. Experimental results from bulk InP should still give reasonable doping estimates of our moderately doped samples. Schwabe et al. measured low-temperature PL of highly Sn-doped InP27 and observed a peak position which was shifted about 100 meV above the band gap at a doping level of 1.5  1019 cm
3. Since our shift at χH2S = 1.9  10
6 is slightly larger and the B-M shift scales inversely with the effective mass, we estimate the carrier concentration at this doping level to 2  1019 cm
3. In contrast, the S concentration from EDX for this sample was measured to 0.6%, i.e., 2  1020 cm
3, but the EDX measurements are complicated by the plausible S terminated surface. As a theoretical comparison, 100 nm long atomic columns terminated with single S atoms at each end would also give a detected S concentration of approximately 0.6%. Clearly EDX cannot reveal the carrier concentration in the interior of these NWs. Note that the PL evaluation shows a more stable increase with much smaller relative error bars than the EDX measurements, indicating its potential as a quantitative doping measurement tool. Estimating the carrier concentration with PL in higher doped samples is difficult since the second CB contributes to the density of states. An estimate can instead be achieved by using the concentration detected by EDX in the highest doped sample, χH2S = 11.1  10
6, and compensating for a monolayer shell of S as discussed above: 0.9%
0.6% = 0.3%. This corresponds to a carrier concentration of 1.2  1020 cm
3, which is also in good agreement with a simple scaling of the carrier concentration with χH2S. Doping levels of 1  1020 cm
3 have been achieved using H2S in thin film growth of InP,28 without indications of saturation. Unlike GaAs, S doping of InP does not show signs of compensation.29 The simultaneous presence of free carriers in two conduction band minimums could lead to unusual electron transport effects. PL of the highest doped sample (χH2S = 11.1  10
6) indicated that a large share of the free carriers was residing in the higher CB. To investigate the electron transport properties, some of these were broken off and laterally contacted as NW-FET devices.1 Electrical measurements displayed Ohmic behavior with total device resistances of 600
700 Ω. Transmission line measurements failed to separate the contact resistance from the NW resistance, indicating that the contact resistance was dominating the device resistance. The devices showed no dependence on back-gate bias, as expected for degenerate doping. Temperaturedependent measurements using liquid helium showed no change in conductivity, in agreement with previous measurements on highly doped InP.29 The mobility was estimated using a Drude model to be about 400 cm2 V
1 s
1, assuming a NW resistance of 300 Ω, a channel length of 2 μm, and a carrier concentration of 1.2  1020 cm
3 as discussed above. This mobility is higher than InP NWs doped using TESn,30 despite higher doping, possibly due the perfect crystal structure and the smooth sidewalls. Presumably elastic scattering from ionized impurities dominate the scattering, allowing the mobility to reach values similar to bulk InP at similarly high doping levels.29 The higher effective mass of the

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second CB suggests a lower mobility, but this may be compensated for by reduced scattering due to the higher electron energy. Several devices were purposely driven until failure and reached current densities of 1  107 A/cm2 before destruction, without use of the back gate. These high breakdown currents are an order of magnitude higher than those previously reported for highly doped and strongly gated Si NWs31 and further indicate that elastic processes dominate scattering. In conclusion, we have demonstrated that in situ doping with H2S creates perfect WZ InP nanowires with high S levels. The high electron density creates strong state-filling and band gap renormalization which give characteristic spectra in PL, and which we used to estimate the carrier concentration in the NWs. For nanostructures grown in the bulk crystal structure for which accurate reference results are available, the B-M shift could give excellent doping estimates with a processing-free and relatively simple technique. We used the state filling to probe the band structure of a material which is unavailable in bulk and found evidence of a closely separated second CB in agreement with theoretical predictions.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed description of methods used and PLE spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Bo Monemar and G€unther Grossman for fruitful discussions and Jun Wu for PLE measurements. This work was performed within the Nanometer Structure Consortium at Lund University (nmC@LU) and was supported by the Swedish Research Council, by the Swedish Foundation for Strategic Research, and by the EU program AMON-RA (214814). This report is based on a project which was funded by E.ON AG as part of the E.ON International Research Initiative. ’ REFERENCES (1) Cui, Y.; Lieber, C. M. Science 2001, 291 (5505), 851–853. (2) Minot, E. D.; Kelkensberg, F.; van Kouwen, M.; van Dam, J. A.; Kouwenhoven, L. P.; Zwiller, V.; Borgstr€om, M. T.; Wunnicke, O.; Verheijen, M. A.; Bakkers, E. P. A. M. Nano Lett. 2007, 7 (2), 367–371. (3) Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L. F.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B. J.; G€osele, U.; Samuelson, L. Mater. Today 2006, 9 (10), 28–35. (4) Hiruma, K.; Yazawa, M.; Katsuyama, T.; Ogawa, K.; Haraguchi, K.; Koguchi, M.; Kakibayashi, H. J. Appl. Phys. 1995, 77 (2), 447–462. (5) Mishra, A.; Titova, L. V.; Hoang, T. B.; Jackson, H. E.; Smith, L. M.; Yarrison-Rice, J. M.; Kim, Y.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C. Appl. Phys. Lett. 2007, 91 (26), 263104. (6) De, A.; Pryor, C. E. Phys. Rev. B 2010, 81 (15), 155210. (7) Gunn, J. B. Solid State Commun. 1963, 1 (4), 88–91. (8) Lu, H. L.; Terada, Y.; Shimogaki, Y.; Nakano, Y.; Sugiyama, M. Appl. Phys. Lett. 2009, 95 (15), 152103. (9) van Weert, M. H. M.; Helman, A.; van den Einden, W.; Algra, R. E.; Verheijen, M. A.; Borgstr€ om, M. T.; Immink, G.; Kelly, J. J.; 2289

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