H-tailored surface conductivity in narrow band gap In(AsN)

The incorporation of low concentrations (∼1%) of nitrogen-atoms in III-V semiconductor compounds acts to modify the electronic band structure of the host crystal, thus enabling band structure engineering and a fine tuning of fundamental electronic properties, such as the band gap energy.^1,2 An additional attractiveness of this class of compounds is the possibility of using post-growth hydrogenation, combined with electron beam lithography and masking³ or laser writing,⁴ to create distinct, tailor-made light emitting regions,^3,4 all integrated onto a single substrate. These effects rely on the ability of hydrogen to modulate the material optical properties by passivating the electronic activity of nitrogen through the formation of di-hydrogen N-H₂ complexes.⁵ 

Although the physics of hydrogen in wide-band gap dilute nitrides, such as Ga(AsN), has been researched both experimentally and theoretically, the effects of hydrogen on the electronic properties of narrow band gap III-N-Vs, such as In(AsN), are still largely unknown.^6–8 The mechanisms of hydrogen diffusion, H-N interaction, and, in particular, the passivation of the electronic activity of nitrogen by hydrogen, which is well established for wide bandgap III-N-Vs,³ can be qualitatively different in narrow band gap compounds and may open perspectives in the exploitation of these materials in mid-infrared optoelectronics.⁹ In particular, the narrow band gap InAs semiconductor has an electron accumulation layer in the surface region with a Fermi level, E_F, located well above the conduction band minimum.^10–12 Hydrogen, being either an unintentional or intentional dopant, can alter surface properties;¹¹ furthermore, the electronic behaviour of monoatomic-H can be modified by nitrogen through the formation of N-H donor complexes.⁸ 

In this Letter, we exploit the combined effects of nitrogen and hydrogen in InAs and show that the n-type conductivity of the In(AsN) alloy can be significantly increased within a thin (∼100 nm) channel below the surface by the controlled incorporation of H-atoms. For a fixed dose of impinging H-atoms, the width of the surface conducting channel decreases with the increase in concentration of N-atoms, which act as H-traps, thus forming N-H donor complexes near the surface. The channel retains a high electron mobility (μ > 0.1 m²V⁻¹s⁻¹ at low and room temperature) with an electron sheet density of 10¹⁸m⁻², significantly larger than in N-free InAs. This controlled modification of the surface conductivity and large increase in the electron sheet density by hydrogen could provide a platform for several applications, such as gas and molecular sensing¹² and “all semiconductor” plasmonic waveguides.¹³ 

The nominally undoped In(AsN) epilayers (thickness d = 1.0 μm and N-content, [N], of 0, 0.15, 1.1, and 1.9%) were grown by Molecular Beam Epitaxy (MBE) on a semi-insulating (SI) (100)-oriented GaAs substrate, which provides effective isolation for electrical measurements. The wafers were spun with photoresist and patterned by standard photolithography into Hall bars. Metal contacts consisting of 40 nm of Ti followed by 400 nm of Au were deposited onto the samples to provide Ti-Au ohmic contacts. Hydrogen was implanted in the epilayers using a Kaufman source with ion current densities of a few tens of mA/m² and an ion-beam energy of 100 eV for a time t_H ranging from 9 min to 2 h, impinging H-doses D_H = 10²⁰ ions/m² ([N] > 0) and D_H = 3 × 10²¹ ions/m² ([N] = 0), at temperatures T_H = 250–300 °C. As shown later, these relatively low H-doses enable us to incorporate hydrogen near the surface of the In(AsN) epilayers.

To investigate the H-diffusion profile, the epilayer with [N] = 1.1% was irradiated with deuterium, ²H, for which Secondary Ion Mass Spectrometry (SIMS) has a higher sensitivity. A Cs⁺beam with energy of 1 keV incident at 55° on the sample surface was used in a CAMECA Sc-Ultra mass spectrometer and negative secondary ions (²H⁻, ⁷⁵As⁻, ¹¹⁵In^–, and ⁷⁵As¹⁴N⁻) were collected. For the magneto-transport studies, the magnetic field B was generated either by a superconducting magnet (up to 14 T) or a room temperature electromagnet (up to 1 T). The magnetic field was applied parallel to the growth axis z, i.e., B = [0, 0, B], or at an angle θ relative to z.

Figure 1 shows the second derivative of the transverse magnetoresistance, d²R_xx/d²B, as a function of the inverse magnetic field, 1/B, for both virgin (V) and hydrogenated (H) In(AsN) epilayers with [N] = 0.15% at T = 2 K. For the V-sample, we observe Shubnikov-de Haas (SdH) oscillations at B > 1 T with period Δ_L(1/B) = (0.26 ± 0.02)T⁻¹. Following the hydrogenation, an additional series of SdH oscillations emerge at B > 3 T with a shorter period Δ_S(1/B) = (0.012± 0.001)T⁻¹. These short-period oscillations are not observed in N-free InAs (see inset of Fig. 1) and persist for B tilted at various angles from θ = 0° to 90°. The positions in 1/B of the maxima/minima in d²R_xx/d²B do not scale as 1/Bcosθ, thus indicating that they are caused by the SdH effect in a 3D electron gas. The periods Δ_S and Δ_L give 3D electron densities of $n=(2e/ℏΔ)3/2/(3π2)$ = (4.2 ± 0.5) × 10²⁴m⁻³ and (4.3 ± 0.5) × 10²²m⁻³, respectively.

All N-containing epilayers hydrogenated under the same conditions (D_H = 10²⁰ ions/m², t_H ≈ 9 min, and T_H= 250–300 °C) exhibit a similar behavior, i.e., hydrogen induces an additional series of short-period SdH oscillations (see Figure 2(a)). With [N] increasing from 0.15 to 1.9%, these oscillations emerge at larger B, which corresponds to a decrease of the electron mobility, and the period Δ_S decreases from 0.012 to 0.0055 T⁻¹, which implies an increase in the electron density n_s from 4.2 × 10²⁴m⁻³ to 1.4 × 10²⁵m⁻³. These electron densities are almost two orders of magnitude higher than those found in the virgin samples and those derived from the long-period SdH oscillations, which vary from n_b = 4.3 × 10²²m⁻³ to 5.9 × 10²³m⁻³, see Figure 2(b). The H-induced increase in the electron density tends to saturate to about 10% of the total N-concentration with H-doses increasing from D_H = 10²⁰ to 10²³m⁻², see Figure 2(b). Correspondingly, the Fermi energy E_F remains pinned to a value in the range 0.2 to 0.4 eV above the conduction band minimum of In(AsN).¹⁴ 

Given the observation of SdH oscillations with two distinct electrons densities, we use a conduction model with two parallel channels¹⁵ to determine the width of each channel and its contribution to the overall Hall resistance, R_xy, given by

where d_s is the thickness of the surface channel, $db=d−ds$⁠, d is the thickness of the epilayer, and $σs(b)$ and $μs(b)$ are the electron conductivity and mobility of the surface (s) and bulk (b) layers, respectively. Figure 3(a) shows least-square fitting of Eq. (1) to the Hall resistance curves, R_xy(B), for [N] = 0.15, 1.1, and 1.9%, each reproducing accurately the non-linear B-dependence of R_xy. Each fit uses only one fitting parameter, d_s, with all other parameters fixed to the values measured experimentally: The carrier densities, n_s and n_b, of the two channels are derived from the measured period of the SdH oscillations; the mobilities, μ_s and μ_b, are obtained from the condition μB > 1 required to observe each set of oscillations; and the epilayer thickness d is set to the nominal value of 1 μm (see Table I). The same set of parameters was used to fit the values of the transverse resistance at B = 0 T, i.e., $Rxx(0)=LW−1(σsds+σbdb)−1$⁠, where L = 750 μm and W = 250 μm.

Our data analyses indicate that d_s decreases from 187 to 77 nm with increasing [N] from 0.15 to 1.9%, from which we infer a smaller penetration depth of H at higher [N] and an almost constant electron sheet density (n_sd_s ≈ 10¹⁸m⁻²). As shown in Figure 3(b), the SIMS profile for ²H has a box-shaped form from which we estimate a ²H penetration depth of ∼100 nm for [N] = 1.1%. This is close to the value of d_s = 128 nm inferred from the magneto-transport data. Thus, we propose that under the small H-doses and relatively low hydrogenation temperatures used in this work, H-atoms tend to be effectively trapped by the N-atoms in a thin (≈100 nm) subsurface layer. In this region, the H-atoms can strongly bind to nitrogen and form H-N donor complexes H(BC_N), in which H occupies the bond-center site between a N- and an In-atom (see inset of Figure 2(b)).⁸ 

We estimate the resistance, R_S, of the surface channel to be R_S = $(ensμs)−1L/(dsW)$ = 46, 97, and 158 Ω for [N] = 0.15, 1.1, and 1.9%, respectively. These values are comparable to the total measured resistance (R = 42, 82, 58 Ω) and can be significantly smaller than the resistance of the bulk layer, i.e., R_b = $(enbμb)−1L/(dbW)$ = 440, 546, and 90 Ω. The smaller value of R_b for [N] = 1.9% is due the larger bulk electron density, n_b (see Figure 2(b)). Thus, the contribution of the H-containing subsurface layer to the conductivity is significant and can be dominant even in thick (∼1 μm) epilayers. This is in stark contrast to the behavior of N-free InAs. From the analysis of the Hall resistance using the two channels model, we find that the H-induced increase of the electron density is negligible and that the surface accumulation layer has an electron sheet density of ∼10¹⁶m⁻², significantly smaller than for In(AsN). Furthermore, whereas in InAs the charge is located on the surface thus leading to a bending of the bands and the formation of two-dimensional (2D) accumulation layer,¹⁶ in In(AsN), the spatially uniform density of donors across the thin (∼100 nm) subsurface region of the epilayer and large electron sheet density lead to a 3D electron gas near the surface (Figure 3(c)).

We note that despite the incorporation of H and the relatively large electron sheet density (≈10¹⁸m⁻²), the In(AsN) surface channel has a relatively high mobility (>0.1m²V⁻¹s⁻¹) at both low and room temperature; also, the H-containing In(AsN) layer retains a high electron sheet density after many thermal cycles from T = 4 K to 300 K, and thermal annealing in argon at temperatures up to 473 K, thus indicating a stable modification of the In(AsN) alloy by hydrogen, an essential requirement for future applications of this compound.

In summary, we have shown that the n-type surface conductivity of In(AsN) is modified by the controlled incorporation of H-atoms through the formation of donors within a well-defined subsurface region of the In(AsN) epilayer. For a fixed H-dose, the width of this channel decreases with the increase in concentration of N-atoms, which act as effective barriers to the H-diffusion thus producing box-shaped ²H-diffusion profiles and a high electron sheet density (∼ 10¹⁸m⁻²). Our findings will stimulate theoretical studies of H-diffusion and energy levels in mid-infrared III-N-Vs, where hydrogen, being either an unintentional or intentional dopant, affects surface electronic properties.¹¹ Furthermore, our results open up perspectives for employing mid-infrared In(AsN) in applications that exploit its n-type surface conductivity, ranging from gas sensing¹² to “all semiconductors” plasmonic waveguides¹³ for mid-infrared photonics. In particular, the controlled photodissociation of H-N donor complexes by a focused laser beam⁴ or the selective H-incorporation by masking may provide a route for writing conducting paths and printed circuits on a single substrate.

This work was supported by EPSRC.