DD279952A1 - METHOD FOR DETERMINING ELECTRICAL AND OPTICAL KNOWLEDGE OF SEMICONDUCTOR MATERIALS - Google Patents

Abstract

The invention relates to a method for determining electrical and optical characteristics of semiconductor materials. It includes a test method for semiconductor base materials, in particular compound semiconductors and component structures of microelectronics and optoelectronics. The goal is a non-contact, spatially resolved determination of the majority of the analyzer concentration, electrical barrier height, Grenzflaechenladungs- and density of states and the optical transition energy Ei, the relationship Ei (x) or the composition x in mixed compounds of compact material and layer structures. By means of modulation spectroscopy and light probe technology, the photoreflectance spectrum is measured spatially resolved and evaluated as an electroreflection signal. From the optical transition energy Ei, electro-optical energy hV and amplitude A, the material parameter values are determined. Fig. 3

Description

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Fields of application of the invention

The invention includes a method for the determination of electrical and optical material parameters on semiconductors, which is suitable for controlling in the manufacturing process of semiconductor base materials and component structures of microelectronics and optoelectronics. It can be used for routine testing to quantify targeted electrical and optical material parameters and detect unwanted deviations.

The method can be applied to semiconductor materials with low carrier concentrations, in particular also when investigating semiconductor layers on any substrate. A particularly useful field of application is the characterization of ternary and quaternary mixed crystal materials of the type "A, - _X B _X C" or "A ₁ , B _x C, _y D" in particular in the presence of thin layers.

Characteristic of the known state of the art

In the development of new semiconductor materials and device structures in microelectronics and optoelectronics, as well as in the technological control of the fabrication process, characterization of base materials, wafer structures and device chips in terms of electrical and optical semiconductor parameters such as carrier concentration, optical gap energies, according to the adjusted mixed crystal compositions , as well as the surface and interface properties, i. a. also a sufficiently spatially resolved determination is sought. To determine the charge carrier concentration in the compact material, epitaxial layers and layer systems methods are known which are based on the implementation of electrical measurements (Hall measurements / 1 /, analysis of the capacitance-voltage characteristics / 2 /, measurement dese surface contact resistance / 3 /, measurement the field strength M /). These methods all have the disadvantage that one or more electrical contacts must be prepared. In the simplest case, contact is made by forming a metal tip, but this is associated with local material damage. A measurement with high spatial resolution is i. a. only conditionally possible. For the investigation of epitaxial thin films, a corresponding disadvantage of the electrical processes is that substrate effects must be eliminated. This is z. B. in Hall measurements on epitaxial layers an insulating substrate ahead; However, in the case of epitaxial layer systems for device structures, this requirement can often not be met. Optical methods for charge carrier concentration determination are the photoluminescence, Raman and plasma reflection spectroscopy / 5,6 /, but they are limited in their scope to higher carrier concentrations. Electroreflection / 15 / requires as an optical modulation method an external electric field i.a. is introduced by means of an electrolyte electrode, which, however, then irreversibly influences the surface state by interfacial reactions.

The characterization of the surface and interface properties of the semiconductor material is u. a. by determining barrier heights, state densities, and interface recombination rates in the surface layers and internal transition region. As a method to z. B. Katodolumineszenzuntersuchungen / 7 / and electron beam induced barrier current measurements / 8 / and scanning electron microscope and the electrical

Current-voltage or capacitance-voltage characteristic investigations / i) / known. While the characteristic measurements are the o. G. Disadvantages of electrical methods, the scanning electron microscopy measurements and the analysis of their measurement data are time consuming and limited in their applicability with decreasing layer thickness of the semiconductor material. The connection between the optical transition energies and the mixed crystal composition, which is important for the semiconductor mixed crystal materials, can be determined by the use of known, complementary methods, such as the electron beam microanalysis / / / and the usual optical spectroscopic methods / 5, 6 /. Limitations of these methods arise for the case of very low layer thicknesses on the one hand or due to the high demands on the performance parameters of the spectroscopic arrangements on the other hand.

Object of the invention

The aim of the invention is the necessary for the technology optimization of the development and manufacturing process diagnosis of correlated electrical and optical material parameters of semiconductor base materials and complete device structures in microelectronics and optoelectronics with special consideration of arising from the use of compound semiconductors and layer structures issues by a method, characterized by simple, non-destructive and time-economic application conditions, a hitherto unavailable complexity, an extended range of application that is not covered by other methods, as well as high accuracy and reliability.

Explanation of the essence of the invention

The invention is based on the object, a method for berühnmgslosen and spatially resolved determination of the charge carrier concentration, the electrical Barrierenhche and state densities of surface and interface areas, and the optical transition energies and in the case of mixed compound semiconductors derived therefrom mixed crystal ratio of compact materials, thin films, hetero structures and layer systems microelectronic and optoelectronic semiconductor base materials and device structures.

According to the invention the object is achieved in that determined by means of a light probe technique, the photoreflective (PR) spectrum according to the principle of modulation spectroscopy / 10 / in the relevant semiconductor material by the position of the optical transitions at the direct gap and at further, higher energy transitions Spectral range is measured. From the measured spectra, the energetic layers of the optical transitions, in a known manner as E ₀ . Ei, ..., etc., as well as the amplitudes of the spectral structures, broadening parameters and in particular the periods of Franz Keldysh oscillations / 11 / derived.

Figure 1 shows the physical model of a surface barrier structure in the case of the depletion boundary layer in an η-type semiconductor. The PR signal originates maximally from the area of the space charge width (d _f ). Fig. 2 shows the scheme of the measuring arrangement and illustrates the measuring principle of the optical modulation reflection spectra. From the sample surface

(1) the measuring light (2) originating at the light source (3) and passing through the monochromator (4) is reflected and registered with the radiation receiver (5). for the measurement of the reflection spectrum R a Gleicnlichtsignal, in the case of the measurement of the modulated reflection AR registers a alternating light signal with the reference frequency of the laser light change (7) and the quotient AR / _R forms.

; The modulation here causes the periodically incident on the sample laser light beam whose quantum energy must be greater than the optical gap energy E ₀ .

By separation of the generated electron-hole pairs in the Ba; ration field, the electrical edge field strength is varied. Measuring light and modulation light are optically focused in the sense of a light probe technique by using a light microscope (8) and corresponding light optics (9).

Fig. 3 illustrates a PR spectrum at the E ₀ and Ε, transition. The PR spectrum is in the true sense a photoinduced electro-reflection, ie spectrum and can be described according to the theory of electroreflection / 11 /.

It is: - = α · Δε, + β · ει R

Here ε ,, ει the real part and imaginary part of the dielectric constant ε and α, β represent the Seraphin coefficients / 12 /. In the case of PR spectra, the change in the dielectric constants

B '

S O),

where h is the photon energy of the measuring light and J ^ is the electric fringe field strength, ξ, is given in the context of the physical η model of the depletion boundary layer structure

^ = (2 "V _s / _e 8 ₀ '' ^{/ 2} (1)

Here, e is the elementary charge; n * is the difference of the ionized donor and acceptor concentrations, which is approximately equal to the majority charge carrier concentration. Vs is the electrical barrier height.

The spectra at the E ₀ and E, transition differ. While the E ₀ spectrum is a so-called "high-field" spectrum, the E, spectrum is a so-called "low-field" spectrum. In the first case we have> τ / 3, in the second case <T / 3.

Where T is the broadening parameter for the corresponding transition and

² n ^z Ζ

t ² _Q n ^z / Q / x _n

the electro-optical energy.

Apart from the physical constants e and h, the effective charge carrier mass is parallel to the field direction μ ,, here. By approximation one obtains for the spectral course of the spectra:

AR

"High-field" spectrum

low-field spectrum - jf = C _s · ^ne L ° * ^BV ^ "^ i ^ ^ '' J (4)

C, Θ, k = const., Egg energy of the corresponding electronic. Transition. This results in the following important relationships for the semiconductors-relevant quantities: For the extreme values of the oscillation structure of the "high-field" spectrum applies

o ⁺ 3 Λ 52 ⁽⁵⁾

Z = denominator of the extreme value (0,1,2, ...) or zeros ('/ 2, ³ h, ... ), From which the value for hii is to be determined, which according to (2) the surface field strength ξ, certainly. Ei is determined by the energetic position of the extreme value for Z = 0. The relation (1) gives the relation of ξ, to the carrier concentration π * and barrier height V _s , on the other hand ξ _{5 is} correlated with the surface charge density Q ₅ :

Q »= -εε ,, ξ, (6)

The relationship with the density of surface / interface states is given by

Qss = e / lNS, + N?,) R dE] (7)

nt; ^D - Density a '<zeptor- or donator-like surface states

f "- occupation function for electrons

E ₅₅ - energetic position of surface states

Ef-Fermienenergie

In the case of the "low-field" spectra there is an "amplitude" proportional to the square of the electric field strength:

Ä = T ^k · (- ^ - J _A · f (P _A ) = T ^k · (- ^ -) a · «P _B > ~ S (8)

η η

A and B denote dominant extreme values of the energetic layers E _A and E _u in the spectral structure. The transition energy Ei and the propagation parameter τ are determined by means of 3-point method / 13 / according to (9).

Ε, = E _A + (Eb - E _A ) · f (p); T = (E ₈ -E _A > G (P) O)

Where P is the asymmetry parameter and f (P) and g (P) are known for the different transition types

Correction functions / 13 /.

For ternary mixed crystal materials A _x B, _X C is due to the known transition energies due to known or by

Calibration measurements to determine correlations

E ₁ (X) = Ei (BC) + X [E ₁ (AC) - Ei (BC)] - X ² E ₁₃ , (10)

E _B - "bowing" parameter

the y-value available. The determination of the mixed crystal composition is also applicable to quaternary mixed crystals. The determination of the semiconductor parameters is based on the quantitative analysis of the PR spectra according to (1) and (10). It should be pointed out that because of ξ ² ~ n * V _s according to (1) the absolute values for n * and V _1, respectively, require the knowledge of the other variable, and for the assumption of the magnitude for V _s this is the case for many semiconductors known Ferminiveaupinning ar surfaces and interfaces can be used. Relative changes are detected directly with high sensitivity. PH spectra generally require only lower spectral resolution and can be measured with high lateral resolution (~ 10pm). An essential feature of the method is the low level of information, which is determined by the minimum penetration depth of the measuring light in the region of the respective transition energies and penetration depth of the electrical fringe field. It can be down to 10nm (eg, Ε, transition to GaAs). This also covers layers in the submicrometer thickness range.

An optically transparent cover layer allows PR measurements at the semiconductor interface, so that mixed crystal compositions and charge carrier concentrations or barrier height can be determined in the semiconductor material of a semiconductor-metal or semiconductor-insulator system or in a semiconductor heterostructure, the measurement method allows a favorable coupling with Slope grinding or layer removal methods.

embodiment

Given is a) a semiconductor sample consisting of a substrate and one or more homo- or heteroepitaxial layers and bj a semiconductor sample with partially free surface and a semitransparent metal contact or a transparent insulator layer or MOS / MIS structure, the surface b) also on the sample a) could be located (Fig.4 shows sketches for concrete examples).

To be determined is the mixed crystal composition of the heteroepitaxial layers as well as the charge carrier concentration of all layers; in addition, the barrier height at the interfaces of the heterostructure is investigated.

In the case b) so ¹ 3arrierenhöh? the metal-semiconductor contact and the band bending are determined under the insulator layer, fer. -oil the influence of different surface preparations on the surface state density can be detected. To investigate a sequence of layers, the polish-grade bevel is to be used as the measurement surface in order to make the inner layers accessible to the PR measurements. Alternative ways are gradual layer removal z. B. by electrolytic oxidation or sputtering.

The task is solved by measuring the PR spectra in the spectral range of the expected transitions at the measurement locations 1 to 4. From each spectrum, the values Ej and the electro-optical energy hil or the amplitude Ä can be determined; For this, use relations (5), (8), (9).

The determination of the mixed crystal composition takes place according to (10) from the transition energies E ₁ . The carrier concentration is determined by means of the relationships (2) and (1) directly or with (8) and (1) using calibration measurements. The required knowledge of Oberflächenbandverbiegung is given for many semiconductor materials with sufficient accuracy for these purposes or can be set by standard preparation or determined by independent measurements.

By line scan 3 -> 4 under measurement of the peak height

() 'at fixed wavelength, the gradient of the carrier concentration can be determined. In the same way you can

relative doping inhomogeneities are examined laterally.

Full spectra only need to be measured at selected locations.

At the points 2, 3 as well as at low cover layer thickness at 1, E ₀ -PR-spectra can also be measured by partial reflection at the next lower boundary surface of the deeper layer. From this and with the previously determined charge carrier concentration of the corresponding layer, the band bending at the inner boundary surface is determined.

Similarly, the barrier height of 2 and 3 is to be determined in Example b, E ₀ -PR measurements are more favorable, since the line shape is compared to the amplitude at higher transitions, which must be corrected in existing cover layers evaluated.

In the investigation of the influence of surface preparations or surface reactions on the surface state, the change in the total PR spectra or their peak height at a fixed wavelength is monitored accordingly. From this the corresponding changes of Q ₅ , and V _{5 are} determined and with the help of models for the energetic position of surface states E _S5 / 14 / their density N ₅ , with (7). Temperature-dependent PR measurements yield Q ₅₅ (T) and Vs (T), so that such energy levels can be determined or specified taking into account the temperature dependence of the population function f "(E ₅₅ , E _F ).

Gate voltage-dependent photoreflective or electroreflectance measurements V ₅ (Ug) allow investigations of the spectrum of surface states N ₅₅ (E) according to the evaluation of CV measurements.

Claims (4)

1. A method for determining electrical and optical characteristics of semiconductor materials, characterized in that with the aid of the Lichtsondentechnik by measuring the photoreflective spectra according to the principle of modulation spectroscopy on the semiconductor material, in particular on Verbindungshalbleilern, the optical transition energies Ej and from the analysis of the spectral structure of the value of the electric Randfeldstärke ξ _{3 are} determined from which on the basis of a known relationship E, (x) the mixed crystal ratio "x" is derived, and assuming the knowledge of the barrier height in the surface or interface area, the volume carrier concentration is determined or at a given charge carrier density the value of the barrier height and the dam.t correlated interface state density is concluded. 2. The method according to claim 1, characterized in that it is used as a non-contact and thus non-destructive and spatially resolving method for determining the electrical or optical material parameters in a broader scope both of semiconductor base materials as well as complete device structures in the form of wafers and Chips, whereby further use of the material under investigation can be ensured. 3. The method according to claim 1 and 2, characterized in that it is applied to compact semiconductor material, in particular to Mischkristallverbindu / igen, epitaxial layers, possibly incorporating the Schrägschlifftechnik or a elektroiytischen Schichtabtrages. 4. The method according to claim 1 to 3, characterized in that it is used to detect the local and temporal changes in properties of base materials and components.