A method for determining a layer thickness of a sample during processes causing a shrinking or growing of said layer thickness, the method comprising: a) providing the sample inside a processing chamber, wherein the sample comprises a semiconducting wafer and wherein the sample is a AlGaN layer or a p GaN layer; b) etching or depositing the sample in the processing chamber, wherein the etching is atomic layer etching and the depositing is atomic layer deposition; c) wherein said the etching or depositing is performed in a cyclic mode, d) wherein during the cyclic etching or cyclic depositing of the sample, the following steps e) to i) are carried out: e) irradiating optical radiation onto the sample, the optical radiation comprising light of a first wavelength (λ1) and light of a second wavelength (λ2) the first wavelength (λ1) being different from the second wavelength (λ2); f) measuring a first reflection signal from the sample with respect to the first wavelength (λ1); g) measuring a second reflection signal from the sample with respect to the second wavelength (λ2); h) calculating a thickness of the sample based on the first reflection signal and the second reflection signal; i) each of the first wavelength (λ1) and the second wavelength (λ2) is selected from a range of 300 nm to 370 nm, wherein j) a look-up table is used for calculating a thickness of the sample, the look-up table including correlation data of the thickness of the sample and respective measured reflectance signals at the first wavelength (λ1), wherein the used look-up table is generated by averaging the measured reflectance data with an averaging time that is identical to cycle time. a) providing the sample inside a processing chamber, wherein the sample comprises a semiconducting wafer and wherein the sample is a AlGaN layer or a p GaN layer; b) etching or depositing the sample in the processing chamber, wherein the etching is atomic layer etching and the depositing is atomic layer deposition; c) wherein said the etching or depositing is performed in a cyclic mode, d) wherein during the cyclic etching or cyclic depositing of the sample, the following steps e) to i) are carried out: e) irradiating optical radiation onto the sample, the optical radiation comprising light of a first wavelength (λ1) and light of a second wavelength (λ2) the first wavelength (λ1) being different from the second wavelength (λ2); f) measuring a first reflection signal from the sample with respect to the first wavelength (λ1); g) measuring a second reflection signal from the sample with respect to the second wavelength (λ2); h) calculating a thickness of the sample based on the first reflection signal and the second reflection signal; i) each of the first wavelength (λ1) and the second wavelength (λ2) is selected from a range of 300 nm to 370 nm, wherein j) a look-up table is used for calculating a thickness of the sample, the look-up table including correlation data of the thickness of the sample and respective measured reflectance signals at the first wavelength (λ1), e) irradiating optical radiation onto the sample, the optical radiation comprising light of a first wavelength (λ1) and light of a second wavelength (λ2) the first wavelength (λ1) being different from the second wavelength (λ2); f) measuring a first reflection signal from the sample with respect to the first wavelength (λ1); g) measuring a second reflection signal from the sample with respect to the second wavelength (λ2); h) calculating a thickness of the sample based on the first reflection signal and the second reflection signal; i) each of the first wavelength (λ1) and the second wavelength (λ2) is selected from a range of 300 nm to 370 nm, wherein j) a look-up table is used for calculating a thickness of the sample, the look-up table including correlation data of the thickness of the sample and respective measured reflectance signals at the first wavelength (λ1),