Electrical characterization of ZnO: an introduction

Electrical characterization of ZnO: an introduction Dr. Ramón Schifano ON. 4.2 Zespół Technologii Nanostruktur Tlenkowych

Outline 1. Personal background 2. Electrical characterization of ZnO -ZnO applications and Ohmic vs. rectifying contacts -Deep level transient spectroscopy (DLTS) -Thermal admittance spectroscopy (TAS) 3. Conclusions and remarks

Personal background Firenze Siracusa

Scientific background - Graduated at the Università degli studi di Firenze (Italy), master thesis title: Evaluation of the doping levels in SiC epilayers by Photoluminescence measurements -1 year post graduated Marie Curie Exchange student at the Linköping Universitet (Sweden) working on GaN photoluminescence measurements -Ph.D degree at the Universitetet i Oslo (Norway): Ph.D thesis title: Shottky contacts and electrical characterization of n-type hydrothermally grown ZnO -Post-Doc at the Micro and Nanotechnology laboratory of the University of Oslo (Norway): working on deposition and characterization of oxides for next generation solar cells -Since 01/02/2014 joined the ON 4.2 division at PAN as a Research associate in electrical measurements of semiconductors and devices

ZnO applications: possible and commercial High efficiency lighting devices for general illumination UV emitters and detectors Space electronics, spintronics Transparent electrodes for solar cells and flat panel displays High temperature, high frequency devices Varistors

Current material related issues p-type doping in ZnO has not been well accomplished the majority of the electrically active defects are not assigned Ubiquitous and/or fast diffuser related defects not clear (see for example H, Zn i ) High quality Schottky and Ohmic contacts are crucial to perform any electrical characterization and for device applications

Ohmic and Schottky contacts to ZnO Ohmic contacts not challenging, Ti based schemes, Al, InGa (~10-7 Ω cm 2 in highly doped ZnO (1) ) Schottky contacts are more challenging to realize Pd, Ag, Au, Pt, Ir investigated in - solvents precleaning (2) - annealing in oxygen plasma (3) - chemical treatments in H 3 PO 4 + HCl (4) or H 2 O 2 (5,6,7) - Hydrogenation/H implantation (8,9) (1) J.-J. Chen et al.. Appl. Phys. Lett. 88, 122107 2006 (2) Polyakov et al. Appl. Phys. Lett. 83, 1575 (2003) (3) Coppa et al. Appl. Phys. Lett.82, 400 (2003) (4) Neville et al.j. Appl. Phys. 41, 3795 (1970) (5) Kim et al. Appl. Phys. Lett. 86, 112101 (2005) (6) R. Schifano et al. Appl. Phys. Lett. 91, 193507 (2007) (7) Gu et al. Appl. Phys. Lett. 90, 122101 (2007) (8) Kim et al. Appl. Phys. Lett. 86, 022101 (2005) (9) R. Schifano et al. Phys. Stat. Sol (c) 205, 1998 (2008)

Schottky contacts to hydrothermally grown ZnO 1) Pd contacts deposition 2) Post deposition annealing might be required (at 200 o C in air for 30 min) AFM amplitude image after organic solvents cleaning 3) Ohmic back contact (InGa) 1) Organic solvent cleaning 2) Dipping or boiling in H 2 O 2 depending on the face Schottky contacts must be deposited AFM amplitude image after 15 min in H 2 O 2 (dipping) R. Schifano et al. Appl. Phys. Lett. 91, 193507 (2007)

H 2 O 2 effects on the electrical behaviour of palladium Schottky contacts current density (A / cm 2 ) current (A) 10 1 10 0 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-2 10-3 10-4 10-5 10-6 -1 0 1 voltage (V) sample (A) sample (B) -4-2 0 2 voltage (V) Organic solvents cleaning After 15 min in H 2 O 2 R. Schifano et al. Appl. Phys. Lett. 91, 193507 (2007) Up to 9 orders of magnitude in current rectification by H 2 O 2 treatment can be achieved!

The equivalent circuit Schottky contact: Pd ZnO Schottky contact: Pd Instrument equivalent circuit Ohmic contact: InGa Sample equivalent circuit R S has to be known prior to perform C-V, DLTS, TAS

Effect of R S on the measurements 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 S d d S d S d d S d S d s d R C R R C j R C R R R R R R C Z C j R Z 1 1 S R d R 1 C d R S d C C

Prior to any capacitance spectroscopy Thermionic emission regime current (I) versus voltage (V) equal to: I=AJ 0 exp(e(v IR S )/ nk B T )(1 exp( e(v IR S )/ k B T )) G / I = e / nk T 1 B GR S e(v-irs)>>kt ± 6 10 Ω 2 R S = 44 n 5 6

Capacitance spectroscopy: DLTS, TAS on Schottky junctions Principle - Study of the changes in capacitance due to the emission/capture of majority carriers by the electrically active defects present in the depleted region Output - Electrical properties of the defect: apparent capture cross section and energy position in the band gap (Enthalpy) - Defect concentration

Defects: carrier emission/capture Shockley-Read-Hall statistics: -a defect is emitting mainly towards the closest band -capture determined by the Fermi level position (where more carriers are) n-type defects in the upper part of the band gap interact with E C (majority carriers trap)

How to get a transient 1) Steady state: Schottky junction in reverse bias 2) Filling pulse: charging of the defect 3) Back to the initial reverse bias: observing the defect decharging n e D filled = N = βt C( t) 2 T σ exp ( e app D t) ΔH exp kbt C 0 ΔC(t) 2 2 1 x1 x 2 NT C exp( e t) 2 D 2 x d ND N T N D

DLTS principle Measuring the transients at different temperature The DLTS spectra is the result of post measurement calculations Transient is sampled at time t 1 and t 2 and substracted * If t 1 fixed and t 2 varied the maximum will shift (*) After Lang J. Appl. Phys. 45 3023 (1974)

S DLTS 1 t (T)= The DLTS spectra 1 t i t p i p t p ti = ΔC0exp t p i t t ΔC(t,T) w(t t ( e(t)t) w(t t p p ) dt ) dt the weighting function w is establishing the e(t) window e(t) resonant with the weighting function DLTS max Varying t i window changes the value of e(t) resonant An Arrhenius plot for e(t) can be constructed

The choice of the weighting function w Simulated DLTS spectra assuming a single exponential transient (1) (2) w lock-in less selective than w GS4 w lock-in more stable than w GS4 After D. Arberg Ph.D thesis KTH (2001)

Defect properties From an Arrhenius plot of et -2 vs. 1/T Courtesy of V. Quemener

The E 3 level ΔH 0.3eV; 15 16 σ app 10 10 cm 2 Labelled as E 3 Present in most samples independently on the growth 17 3 techniques concentrations up to N D 10 cm Attributed both to V O 1 related defects and to Fe/Ni impurities 2 e D 10 5 10 6 s 1 at 300K It can be picked up during C-V measurements at 300K [1] J. Simpson et al. J. Appl. Phys. 63, 1781 (1988). [2] Y. Jiang, et al. J. Appl. Phys. 101, 093706 (2007).

An example: ordinary DLTS on ZnO DLTS spectra is providing: σ app ; H; NT Annealing, implantation, electron irradiation... can be used to alter N T in establishing the chemical nature of the defect 1,2 References Annealed at 1100-1500 o C for 1 h in air L. Vines E. Monakhov R. Schifano et al. J. Appl. Phys. 107, 103707 (2010).

An annealing study on ZnO Increases with annealing in O 2 at 1100 o C E 2 accounts for ~ 30% of the total Fe concentration measured by SIMS E 2 involves Fe in a configuration enhanced by O-rich conditions such as Fe on Zn site V. Quemener et al. Appl. Phys. Lett. 102, 232102 (2013).

Laplace DLTS Large instrumental broadening in ordinary DLTS spectra Resolve time constant ratios larger than ~ 12-15 The spectral density of the transient F(s) can be determined by inverting ΔC(t)= F(s)exp( st ) ds Requires isothermal averaging to achieve a "good enough" signal to noise ratio Possible to study the intrinsic broadening Resolve time constant ratios larger than ~ 2 After L. Dobaczewski et al. J. Appl. Phys. 76 194-198, 1994

The Laplace DLTS spectra Isothermal measurements several Laplace DLTS spectra are needed to built up the Arrhenius plot Laplace spectra on ZnO annealed at 300 o C for 1 h in Ar E 4 is has 3 components with in the 10-14 10-12 σ app W. Mtangi et al. J. Appl. Phys. 111, 094504, 2012

An example: Laplace DLTS on SiGe Effect of the local enviroment on the Au acceptor in SiGe Effect of the strain on the Au acceptor in SiGe Laplace DLTS can be thought as a truly spectroscopic technique L. Dobaczewski et al. J. Appl. Phys. 96 4689, 2004

DLTS: summary What can be used for: σ app ; H; T can be extracted for electrically active defects and possible on their enviroment Requirements: N Schottky diodes with good rectification and low leakage current, probed region depend on the diode characteristics N T limited to 0.01%-10% of N D Limitations: Comparative studies needed to establish the defect chemical nature

TAS requirements and capabilities What can be used for: σ app ; H; T can be extracted for electrically active defects and E D for the main donor, complementary to DLTS Requirements: Schottky diodes with less-good rectification, probed region depend on the diode characteristics Limitations: N Less accurate/less sensitive than DLTS, extraction of the defect characteristics more tedious

TAS principle of the measurement Fixed reverse bias Capacitance and conductance measured with different probing frequency f T Temperature is scanned f T >> e D1 (T) the D 1 centers in x 1 are not responding f T << e D1 (T) the D 1 centers in x 1 are responding

TAS spectra (not formal) Pure capacitance=schottky contact: Q and V in phase Capacitance=Schottky contact with defects: Q Q 0 Re( Q e 0 (cos wt j( wt ( T )) ) cos( T) Q 0 cos( wt ( T)) sin wt sin( T)) Delayed defects response By lowering the temperature it is observed: 1) a drop in the capacitance with at T (f t ) ΔC/ 2 2) a peak in the conductance/ f T at T(f t )

TAS spectra : an example on ZnO E 3 is responding at 250 khz main donor freeze out ΔC - From N T can be extracted R. Schifano et al. Physica B 404 4344, 2009 - N E3 ~2 10 16 cm -3 N D2 ~ 4 10 15 cm - 3

TAS: Arrhenius plot 2f 2 T ( T) T T max 2f T 3 ΔH exp kbt 2 max ΔH exp kbt deeper defects freeze out main donor freeze out

Conclusions On hydrothermally grown ZnO Schottky with high rectification ratios can be obtained by H 2 O 2 treatment Particular attention has to be put in analyzing the Schottky contact characteristics prior to perform any TAS, DLTS σ app ; H; By TAS and DLTS for electrically active defects NT can be extracted TAS and DLTS present complementary characteristics

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Current (A) Effects of H 2 O 2 on the electrical behaviour of the bare ZnO O-face 10-4 10-7 10-10 10-13 Samples A, B organic solvents cleaning Samples A', B' H 2 O 2 treated Bare surface contacted with a metallic probing tip. A dramatic decrease in the current is observed after the H 2 O 2 treatment -2 0 2 Voltage (V) formation of an insulating layer and reduction of the surface leackage current R. Schifano et al. Appl. Phys. Lett. 94, 132101 (2009)

XPS intensity (a.u.) Effects of H 2 O 2 on the Pd/ZnO interface: angle resolved XPS results Sample B ' a.o.e. 0 o a.o.e. 60 o increased PdO formation in the H 2 O 2 treated sample B' respect to the untreated one A' ϕ M is expected to be up to ~2 ev higher for PdO compared to Pd Sample A ' a.o.e. 0 o a.o.e. 60 o 342 341 340 339 Binding energy (ev) R. Schifano et al. Appl. Phys. Lett. 94, 132101 (2009) the formation of PdO increases the SCs barrier height

XPS intensity (a.u.) XPS intensity (a.u.) Effects of H 2 O 2 on the bare ZnO O-face: angle resolved XPS results 0 o a.o.e. Sample C Zn 2p 3/2 O1s Sample C O1s Sample C' 535 530 Binding energy (ev) (b) (a) Sample C Zn L 3 M 45 M 45 an increase in the high energy shoulder of the O 1s peak an increase of the [O]/[Zn] ratio from 1.01 ± 0.05 to 1.11±0.05 Sample C' Zn 2p 3/2 0 o a.o.e. 60 o a.o.e. Sample C' Zn L 3 M 45 M 45 an upward band bending of (0.4±0.1) ev 1025 1020 500 490 Binding energy (ev) R. Schifano et al. Appl. Phys. Lett. 94, 132101 (2009) H 2 O 2 promote the formation of O rich surface layer