Attainment of high-quality Schottky contacts is imperative for achieving efficient gate modulation in heterostructure field effect transistors (HFETs). We present the fabrication methodology and characteristics of Schottky diodes on Zn-polar BeMgZnO/ZnO heterostructures with high-density two dimensional electron gas (2DEG), grown by plasma-assisted molecular beam epitaxy on GaN templates.
Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.
Heterostructure field effect transistors (HFETs) based on two dimensional electron gas (2DEG) have a promising potential for the applications in high speed electronic devices1,2,3. Zinc oxide (ZnO) as a wide bandgap (3.4 eV) semiconductor with high electron saturation velocity has gained considerable attention as a platform for HFETs4,5. Conventionally used barrier material MgZnO ternary necessitate a very high Mg content (>40%) grown at low substrate temperatures (300 °C or lower)6,7, and as such these structures are apt to degrade under high power operations and during thermal treatments, even if the unwanted charge density in the barrier is low enough for gate modulation. To circumvent this obstacle, we have proposed and adopted BeMgZnO as the barrier, in which the strain sign in the barrier can be switched from compressive to tensile via the incorporation of beryllium (Be), making the spontaneous and piezoelectricpolarizations to be additive. As a result, high 2DEG concentration can be achieved with relatively moderate Mg content. Utilizing this approach, high 2DEG densities is observed near the plasmon-LO phonon resonance (~7×1012 cm-2) in BeMgZnO/ZnO heterostructures while the Mg content below is 30% and the Be content is only at 2~3%8.
Due to its similar crystal symmetry, UV and visible light transparency, robust physical and chemical properties, and low cost, c-plane sapphire is widely employed for epitaxy of both GaN and ZnO. Thanks to the remarkable progress achieved in the growth technology of GaN-based electronic and optoelectronic devices on saphhire, high quality GaN templates can be easily produced on sapphire substrates by using AlN or low-temperature (LT) GaN buffer, despite its large lattice mismatch of 16% with sapphire9. Epitaxial growth of ZnO, which has an even larger in-plane lattice mismatch of 18% with sapphire, is relatively well understood for O-polar variety, while the growth of Zn-polar material in two-dimensional mode is not well established. Due to the moderate lattice mismatch of 1.8%, epitaxy of ZnO on GaN is an attractive alternative.
Both MOCVD and MBE are the most successful semiconductor deposition techniques for fabricating high-quality thin films and heterostructures with high reproducibility. The main reason that MBE is less popular than MOCVD for epitaxy of GaN is the cost and inadequacy for mass production. The growth rate in GaN by MOCVD can be several micrometers per hour, and tens of 2 inch (50 mm) diameter wafers or those as large as 6-8" can be grown in one run9. Here, we also adopt MOCVD for the growth of GaN in our study. For the growth of ZnO-based heterostructures, however, more reports on the formation of 2DEG are realized by MBE at the present time prior to the commercialization of the potential applications10,11,12. Recently, we have developed MBE growth of high quality ZnO heterostructures with an accurate control of surface polarity on Ga-polar GaN templates13. It was found that with Zn pre-exposure treatment, ZnO layers so grown exhibited Zn-polarity when nucleated with low VI/II ratios (<1.5), while those nucleated with VI/II ratios above 1.5 exhibited O-polarity. To avoid parallel conduction channel through GaN templates, we adopted carbon compensated semi-insulating GaN MOCVD grown under low-pressure conditions on AlN buffer for the subsequent growth of ZnO-based HFET structures.
Prior to our work14, there has been no reports on the investigation of Schottky diodes on BeMgZnO/ZnO heterostructures. Only several studies have reported on Schottky contacts to MgZnO15,16, e.g., with an ideality factor of 2.37, a barrier height of 0.73 eV, and a rectification ratio of only 103 15. Various Schottky metals have been used for ZnO17, and among them, silver (Ag) has been widely adopted, due to a relatively high Schottky barrier height of 1.11 eV on bulk ZnO with an ideality factor of 1.08 18.
In this work, we aim to fabricate high-quality Schottky diodes for the applications in ZnO-based high-speed HFET devices. The following protocol applies specifically to the fabrication of Ag/BeMgZnO/ZnO Schottky diodes by e-beam evaporation of Ag on the BeMgZnO/ZnO heterostructures grown by plasma-assisted MBE on MOCVD-deposited GaN templates.
1. Growth and Preparation of GaN Template for MBE Growth
2. MBE Growth of BeMgZnO/ZnO Heterostructures
3. Characterizations
4. Fabrication of Schottky Diodes
The left column of Figure 1 shows the evolution of RHEED pattern recorded along the [1-100] azimuthal direction during MBE growth of a Be0.02Mg0.26ZnO/ZnO heterostructure with a 300 nm thick HT-ZnO layer and a 30 nm thick Be0.02Mg0.26ZnO barrier. The right column shows representative surface morphologies at different growth stages (not from the same sample). As evidenced from the appearance of a spotty RHEED pattern, the LT-ZnO buffer layer is of three-dimensional (3D) island growth mode nature. Its surface morphology was improved by thermal annealing treatment at a temperature above 700 °C. It is clearly seen that the surface transformed from 3D to 2D morphology. The subsequent HT-ZnO layer continues to grow in a 2D mode, followed by the 2D growth of Be0.02Mg0.26ZnO layer without the formation of a second phase. AFM measurements have shown that the GaN template has a root mean square (RMS) roughness of 0.28 nm for 5×5 μm2 scan. A smooth surface with an RMS roughness of 0.35 nm is obtained for the HT-ZnO layer without a barrier by growing under O-rich condition and an RMS roughness of 0.45 nm is observed after the growth of BeMgZnO barrier.
HRXRD triple-axis 2θ-ω scan for a typical Zn-polar Be0.02Mg0.26ZnO/ZnO heterostructure with a 300 nm thick HT-ZnO layer, and a 50 nm thick Be0.02Mg0.26ZnO barrier layer is shown in Figure 2. The reflections at 34.46 o, 34.54 o, and 34.75 o are consistent with (0002) reflections of ZnO, GaN, and Be0.02Mg0.26ZnO, respectively. Note that the broadening of the reflection from Be0.02Mg0.26ZnO is due to its thinness. The tensile biaxial strain in the ZnO layer is an indication of the Zn-polar heterostructure, as investigated in our previous study13. Be and Mg contents in the BeMgZnO quaternary were calculated from the Bragg angle of its XRD (0002) reflection and emission photon energy in LT-photoluminescence (LT-PL) spectrum measured at 13 K (not shown).
Figure 3 shows the results of temperature-dependent Hall Effect measurements for a Be0.02Mg0.26ZnO/ZnO heterostructure. The sheet carrier concentration reduced from 8.8×1012 cm-2 to 6.4×1012 cm-2 when the sample was cooled down from room temperature (293 K) to approximately 100 K. With further cooling to 13 K, the sheet carrier concentration saturates at 6.2×1012 cm-2. This finding manifests that the observed reduction in electron concentration is originated from the contributions from parallel conduction channels which include defective nucleation layer and HT-ZnO layer as well as the Be0.02Mg0.26ZnO barrier, if any. This trend has also been reported for MgZnO/ZnO heterostructures10,22. The electron mobility in the Be0.02Mg0.26ZnO/ZnO heterostructure monotonically increases with decreasing temperature; both the 293 K mobility of 206 cm2/Vs and the 13 K mobility of 1550 cm2/Vs are comparable to the values in the literature22,23. The evolution of the electronic properties as a function of temperature clearly indicates the presence of 2DEG at the Be0.02Mg0.26ZnO/ZnO heterointerface.
Figure 4 shows the current-voltage (I-V) curves measured at room temperature for four representative Ag/Be0.02Mg0.26ZnO/ZnO Schottky diodes with a Schottky area of 1.1×10-4 cm2 within one wafer. The forward currents increase exponentially with applied voltage up to 0.25 V, beyond which the voltage drops across the series resistance become apparent. The highest Schottky barrier height of Φap of 1.07 eV was attained with an ideality factor n of 1.22. Rectification ratios of about 1×108 are achieved by using the current values measured at V=±2 V.
Figure 1. Surface characterization. Left column shows the RHEED patterns taken along the [1-100] azimuthal direction during MBE growth of a Be0.02Mg0.26ZnO/ZnO heterostructure, and right column presents the surface morphologies of the GaN template, HT-ZnO layer, and Be0.02Mg0.26ZnO layer measured by AFM. LT-ZnO buffer technology enables the 2D-mode growth of high-quality ZnO heterostructures on low lattice-mismatched GaN templates. Please click here to view a larger version of this figure.
Figure 2. HRXRD of the heterostructure. HRXRD triple-axis 2θ-ω scan of a typical Zn-polar Be0.02Mg0.26ZnO/ZnO heterostructure with a 50 nm thick Be0.02Mg0.26ZnO barrier layer . The reflections at 34.46 o, 34.54 o, and 34.75 o are consistent with (0002) reflections of ZnO, GaN, and Be0.02Mg0.26ZnO, respectively. Please click here to view a larger version of this figure.
Figure 3. Electronic Properties of the heterostructure. Temperature dependences of sheet carrier density and electron mobility of a Zn-polar Be0.02Mg0.26ZnO/ZnO heterostructure. Please click here to view a larger version of this figure.
Figure 4. Schottky diodes. Typical I-V characteristics of four representative Ag/Be0.02Mg0.26ZnO/ZnO Schottky diodes measured at room-temperature. The similarity of the four I-V curves indicates the high in-wafer uniformity of the sample. Please click here to view a larger version of this figure.
Incorporation of BeO into MgZnO to form the quaternary BeMgZnO provides the feasibility to tune the extent and sign of strain in the quaternary and hence significantly increases the 2DEG density8. The representative results show that the Be0.02Mg0.26ZnO/ZnO heterostructure results in a 2DEG density close to the desired plasmon-LO phonon resonance electron density (~7×1012 cm-2)24. Although the electron mobility of the heterostructure strongly depends on the MBE growth parameters such as the substrate temperature and VI/II ratio of both the HT-ZnO and the BeMgZnO barrier layer, the 2DEG density is weakly dependent on the growth conditions and mainly determined by the Be and Mg content in the barrier.
A GaN template is used for the growth of BeMgZnO/ZnO heterostructures with high crystalline quality owing to the moderate lattice mismatch of 1.8% between GaN and ZnO, compared with a large lattice mismatch of 18% between sapphire and ZnO. To avoid any conductive parallel channel, it is critical to have a high resistance in the MΩ/square range for the GaN template. In our case, this is achieved by growing at a low chamber pressure of 76 Torr to enhance carbon compensation. To ensure the polarity control in the BeMgZnO/ZnO heterostructures (Zn-polarity), careful surface treatment of GaN template is indispensable. Any oxidation or contamination introduced during the preparation on the GaN surface would induce Zn- and O- mix-polarity in the heterostructures even the determinant VI/II ratio <1.5 is fulfilled.
Any chemical reaction between the metal and the semiconductor, the presence of surface contaminants, states, defects in the vicinity of the surface, and the diffusion of metal into the semiconductor are common problems in the field of the fabrication of Schottky contacts. A variety of methods has been reported in the literature for preparing the surface of ZnO for Schottky contact fabrication. Among them are etching in HCl (or other acids), physical etching with Ar+, UV Ozone cleaning, treatment in H2O2, and O2 plasma (or mixture with He)25,26,27,28. The etching procedures aim for the removal of a surface layer with thickness a ranging from a few nanometers to microns and therefore cannot be applied for HFET devices. The UV-Ozone cleaning or O2 plasma procedure removes only the surface layer. Therefore, it is well suited for the surface preparation of our BeMgZnO/ZnO heterostructures.
Usually Schottky contacts are achieved by depositing a high work function metal such as Pd, Pt, Ir, etc. In contrast, Ag has a low work function of 4.26 eV. Despite that, devices utilizing Ag electrode can show rectifying behavior owing to the formation of an interface silver oxide layer caused by partial oxidation of Ag with oxygen from ZnO matrix. The so formed oxide layer is transparent for electrons and has higher work function compared to Ag. Raju et al. have reported work functions around 5.5 eV for AgO grown by pulsed laser deposition (PLD), which is 1.3 eV higher than that of Ag, and close to the characteristic of Pd, Pt, and Ir29. Our results indicate that that Ag electrode (with O2 plasma pretreatment on the surface of ZnO heterostructure) is a promising contact metal for the formation of Schottky diodes.
We have demonstrated a method for fabricating high quality Schottky contacts for ZnO-based HFETs. MOCVD grown GaN template with careful surface preparation just prior to MBE growth and a low VI/II ratio <1.5 during ZnO nucleation ensure the Zn-polar orientation of the ZnO-based heterostructures with high quality. MOCVD is a widely-used mature technique for epitaxy of GaN for various applications. The MBE procedure described in this work indicates the combinability of MOCVD and MBE techniques, and GaN and oxide semiconductors for electronic devices. Incorporation of a small amount of Be into the BeMgZnO barrier layer results in HFETs with high 2DEG density, high electron mobility, and high thermal stability, for enhanced high speed performance.
The authors have nothing to disclose.
This work was supported by Air Force Office of Scientific Research (AFOSR) under Grant FA9550-12-1-0094.
MOCVD | Emcore | customer build | |
MBE | SVT Associates | ||
TMAl | SAFC | CAS: 75-24-1 | |
TMGa | SAFC | CAS: 1445-79-0 | |
NH3 | The Linde group | CAS: 7664-41-7 | |
H2 | National Welders Supply Co. | supplier part no. 335-041 | Grade 5.0 |
O2 | National Welders Supply Co. | supplier part no. OX 300 | Industrial Grade Oxygen, Size 300 Cylinder, CGA-540 |
Mg | Sigma-Aldrich | Product No.: 474754-25G | MAGNESIUM, DISTILLED, DENDRITIC PIECES, 99.998% METALS BASIS |
Be | ESPI Metals | Stock No. K646b | Beryllium pieces, 3N |
Zn | Alfa Aesar, Thermo Fisher Scientific Chemicals Inc. | Product No.: 10760-30 | Zinc shot, 1-6mm (0.04-0.24in), Puratronic, 99.9999% |
Au | Kurt J. Lesker | part no. EVMAUXX40G | Gold Pellets, 99.99% |
Ag | Kurt J. Lesker | part no. EVMAG40QXQ | Silver Pellets, 99.99% |
Ti | Kurt J. Lesker | part no. EVMTI45QXQ | Titanium Pellets, 99.995% |
Developer | Rohm and Haas electronic Materials LLC | MF-CD-26 | Material number 10018050 |
Photoresist | Rohm and Haas electronic Materials LLC | SPR 955 | Material number 10018283 |